diff --git a/CHANGELOG.md b/CHANGELOG.md
index ff2b679..48610ec 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,72 @@
 # Changelog
 
+## v4.0.0 — 2024-09-17
+
+### Potentially breaking change: [Default decoded GeoJSON to SRID 4326 (WGS 84)](https://github.com/felt/geo/pull/219)
+
+This aligns our GeoJSON decoding with [the GeoJSON spec](https://tools.ietf.org/html/rfc7946#section-4) by making all decoded GeoJSON infer the WGS 84 datum (SRID 4326) by default. Whereas previously when you called `Geo.JSON.decode/1` or `decode!/1`, we would return geometries with an `:srid` of `nil`, we now return `srid: 4326`. Likewise when encoding GeoJSON, we explicitly output a `crs` field indicating the datum.
+
+This is unlikely to break real-world usage unless your implementation was assuming a different datum by default.
+
+A couple examples of the changes:
+
+**Before**:
+
+```elixir
+iex> Geo.JSON.decode!(%{"type" => "Point", "coordinates" => [1.0, 2.0]})
+%Geo.Point{
+  coordinates: {1.0, 2.0},
+  # Note the old default nil SRID!
+  srid: nil
+}
+```
+
+**After**
+
+```elixir
+iex> Geo.JSON.decode!(%{"type" => "Point", "coordinates" => [1.0, 2.0]})
+%Geo.Point{
+  coordinates: {1.0, 2.0},
+  # New explicit default of WGS 84
+  srid: 4326
+}
+```
+
+If you were to then encode this value again, you'd end up with a new `crs` field in the output GeoJSON:
+
+```elixir
+iex> %{"type" => "Point", "coordinates" => [1.0, 2.0]}
+...> |> Geo.JSON.decode!()
+...> |> GeoJSON.encode!()
+%{
+  "type" => "Point",
+  "coordinates" => [1.0, 2.0],
+  # Note the new `crs` field which was not present in the input to Geo.JSON.decode!/1
+  "crs" => %{"properties" => %{"name" => "EPSG:4326"}, "type" => "name"}
+}
+```
+
+This last behavior is the most potentially troublesome. However, we don't have a good way of distinguishing a case where you explicitly had the `crs` set in the input to the decoding function (in which case you would probably also like to have it present in the re-encoded version) compared to one in which it's been inferred.
+
+Thanks to @gworkman for reporting this issue ([#129](https://github.com/felt/geo/issues/129)).
+
+### Potentially breaking change: [Convert string coordinates to floats, or raise an error](https://github.com/felt/geo/pull/218)
+
+This fixes an issue where we were silently accepting non-numeric coordinates in the GeoJSON decoder, such that you could wind up doing things like decoding a point like `%Geo.Point{coordinates: {"100.0", "-10.0"}}`. This would obviously not have gone well for you later in your processing pipeline, and it violates our typespecs.
+
+The fix here, suggested by @LostKobrakai, is to convert those strings to numbers where we can do so unambiguously. While such inputs are clearly invalid, it's easy enough to handle them in the way that the user was hoping that we should probably just do it. In cases where there's any ambiguity at all, we raise an `ArgumentError`.
+
+### Other bug fixes in v4.0.0
+
+- [Support GeoJSON Feature object with nested GeometryCollection](https://github.com/felt/geo/pull/194) by new contributor @carstenpiepel (🎉)
+
+### Other changes in v4.0.0
+
+- [Fix typo in the README](https://github.com/felt/geo/pull/197) by @caspg
+- [Fix typo](https://github.com/felt/geo/pull/216) by new contributor @preciz (🎉)
+- [Optional dependency bump for `jason` to v1.4.4](https://github.com/felt/geo/pull/215)
+- Dev dependency bumps for ex_doc, benchee, stream_data
+
 ## v3.6.0 — 2023-10-19
 
 As of v3.6.0, `geo` (like [`geo_postgis`](https://github.com/felt/geo_postgis)) is being maintained by the Felt team. As a company building a geospatial product on Elixir, with a track record of [supporting open source software](https://felt.com/open-source), we're excited for the future of the project.
diff --git a/README.md b/README.md
index 1febc84..438c823 100644
--- a/README.md
+++ b/README.md
@@ -33,7 +33,7 @@ _Note_: If you are looking to do geospatial calculations in memory with Geo's st
 ```elixir
 defp deps do
   [
-    {:geo, "~> 3.6"}
+    {:geo, "~> 4.0"}
   ]
 end
 ```
diff --git a/lib/geo/json.ex b/lib/geo/json.ex
index 58352f3..89c9d4d 100644
--- a/lib/geo/json.ex
+++ b/lib/geo/json.ex
@@ -7,13 +7,16 @@ defmodule Geo.JSON do
   so that you can use the resulting GeoJSON structure as a property
   in larger JSON structures.
 
+  Note that, per [the GeoJSON spec](https://tools.ietf.org/html/rfc7946#section-4),
+  all geometries are assumed to use the WGS 84 datum (SRID 4326) by default.
+
   ## Examples
 
       # Using Jason as the JSON parser for these examples
 
       iex> json = "{ \\"type\\": \\"Point\\", \\"coordinates\\": [100.0, 0.0] }"
       ...> json |> Jason.decode!() |> Geo.JSON.decode!()
-      %Geo.Point{coordinates: {100.0, 0.0}, srid: nil}
+      %Geo.Point{coordinates: {100.0, 0.0}, srid: 4326}
 
       iex> geom = %Geo.Point{coordinates: {100.0, 0.0}, srid: nil}
       ...> Jason.encode!(geom)
diff --git a/lib/geo/json/decoder.ex b/lib/geo/json/decoder.ex
index 673bc6a..8deba5e 100644
--- a/lib/geo/json/decoder.ex
+++ b/lib/geo/json/decoder.ex
@@ -96,6 +96,9 @@ defmodule Geo.JSON.Decoder do
       true ->
         raise DecodeError, value: geo_json
     end
+    # Per #129, the GeoJSON spec says all GeoJSON coordinates default to SRID 4326 (WGS 84)
+    # https://tools.ietf.org/html/rfc7946#section-4
+    |> default_srid_4326()
   end
 
   @doc """
@@ -279,4 +282,12 @@ defmodule Geo.JSON.Decoder do
   defp ensure_numeric(other) do
     raise ArgumentError, "expected a numeric coordinate, got: #{inspect(other)}"
   end
+
+  defp default_srid_4326(%{srid: nil} = geom), do: %{geom | srid: 4326}
+
+  defp default_srid_4326(%{geometries: geometries} = geom) when is_list(geometries) do
+    %{geom | geometries: Enum.map(geometries, &default_srid_4326/1)}
+  end
+
+  defp default_srid_4326(geom), do: geom
 end
diff --git a/mix.exs b/mix.exs
index 16be361..2a5146c 100644
--- a/mix.exs
+++ b/mix.exs
@@ -2,7 +2,7 @@ defmodule Geo.Mixfile do
   use Mix.Project
 
   @source_url "https://github.com/felt/geo"
-  @version "3.6.0"
+  @version "4.0.0"
 
   def project do
     [
diff --git a/test/geo/json_test.exs b/test/geo/json_test.exs
index 193be51..90726ff 100644
--- a/test/geo/json_test.exs
+++ b/test/geo/json_test.exs
@@ -36,7 +36,7 @@ defmodule Geo.JSON.Test do
     json = "{\"type\":\"Point\",\"coordinates\":[100.0,0.0,70.0]}"
     geom = Jason.decode!(json) |> Geo.JSON.decode!()
 
-    assert(geom == %Geo.PointZ{coordinates: {100.0, 0.0, 70.0}})
+    assert geom == %Geo.PointZ{coordinates: {100.0, 0.0, 70.0}, srid: 4326}
   end
 
   test "LineString to GeoJson" do
@@ -54,7 +54,7 @@ defmodule Geo.JSON.Test do
     assert(geom.coordinates == {100.0, 0.0})
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson Point without coordinates" do
@@ -64,7 +64,7 @@ defmodule Geo.JSON.Test do
     assert(is_nil(geom.coordinates))
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson with SRID to Point and back" do
@@ -78,7 +78,7 @@ defmodule Geo.JSON.Test do
     assert(geom.srid == 4326)
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to LineString and back" do
@@ -88,7 +88,7 @@ defmodule Geo.JSON.Test do
 
     assert(geom.coordinates == [{100.0, 0.0}, {101.0, 1.0}])
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to LineStringZ and back" do
@@ -100,7 +100,7 @@ defmodule Geo.JSON.Test do
 
     assert(geom.coordinates == [{100.0, 0.0, 50.0}, {101.0, 1.0, 20.0}])
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "Drops M coordinate when decoding LineStringZM" do
@@ -145,7 +145,7 @@ defmodule Geo.JSON.Test do
     )
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to MultiPoint and back" do
@@ -155,7 +155,7 @@ defmodule Geo.JSON.Test do
 
     assert(geom.coordinates == [{100.0, 0.0}, {101.0, 1.0}])
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to MultiLineString and back" do
@@ -167,7 +167,7 @@ defmodule Geo.JSON.Test do
 
     assert(geom.coordinates == [[{100.0, 0.0}, {101.0, 1.0}], [{102.0, 2.0}, {103.0, 3.0}]])
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to MultiLineStringZ and back" do
@@ -185,7 +185,7 @@ defmodule Geo.JSON.Test do
     )
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to MultiPolygon and back" do
@@ -206,7 +206,7 @@ defmodule Geo.JSON.Test do
     )
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to GeometryCollection and back" do
@@ -219,7 +219,7 @@ defmodule Geo.JSON.Test do
     assert(Enum.count(geom.geometries) == 2)
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "Unable to encode non-geo type" do
@@ -281,7 +281,7 @@ defmodule Geo.JSON.Test do
     assert(Enum.count(geom.geometries) == 2)
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJSON to GeometryCollection" do
@@ -454,7 +454,16 @@ defmodule Geo.JSON.Test do
             y <- float()
           ) do
       geom = %Geo.Point{coordinates: {x, y}}
-      assert geom == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
+      assert %{geom | srid: 4326} == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
+
+      geom_with_srid_and_props = %Geo.Point{
+        coordinates: {x, y},
+        srid: 1234,
+        properties: %{"foo" => "bar"}
+      }
+
+      assert %{geom_with_srid_and_props | srid: 1234} ==
+               Geo.JSON.encode!(geom_with_srid_and_props) |> Geo.JSON.decode!()
     end
   end
 
@@ -463,13 +472,13 @@ defmodule Geo.JSON.Test do
     json = Geo.JSON.encode!(geom) |> Jason.encode!()
 
     assert(json == "{\"coordinates\":[],\"type\":\"Point\"}")
-    assert geom == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
+    assert %{geom | srid: 4326} == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
   end
 
   property "encodes and decodes back to the correct LineString struct" do
     check all(list <- list_of({float(), float()}, min_length: 1)) do
       geom = %Geo.LineString{coordinates: list}
-      assert geom == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
+      assert %{geom | srid: 4326} == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
     end
   end
 
@@ -588,4 +597,21 @@ defmodule Geo.JSON.Test do
     assert Enum.all?(geom.geometries, &match?(%Geo.MultiPolygon{}, &1))
     assert geom.properties["id"] == "FLC017"
   end
+
+  defp assert_geojson_equal(%{} = json_1, %{} = json_2) do
+    # Per the GeoJSON spec, GeoJSON is assumed to have WGS 84 datum (SRID 4326) by default
+    assert drop_srid_4326(json_1) == drop_srid_4326(json_2),
+           "Inequivalent GeoJSON values:\n" <>
+             "Left:\n" <>
+             "#{inspect(json_1, pretty: true)}\n" <>
+             "Right:\n" <>
+             "#{inspect(json_2, pretty: true)}"
+  end
+
+  defp drop_srid_4326(%{"crs" => crs} = json)
+       when crs == %{"properties" => %{"name" => "EPSG:4326"}, "type" => "name"} do
+    Map.delete(json, "crs")
+  end
+
+  defp drop_srid_4326(%{} = json), do: json
 end