In May, 2020, DynamoDB was updated to allow storage of empty strings and binaries in non-key fields - documentation. In earlier versions of this app, fields with empty string attributes were stripped off prior to writing to the DB by ExAws.Dynamo.Encodable.do_encode/1; however, in light of these developments, the default behavior of this app will now be to write empty string values.
Please check your application thoroughly and make sure you are prepared for dealing with the possibility of having empty strings in fields that may previously have been expected to return nil (since, in the past, they wouldn't have been written at all). To be clear, existing data is not going to be changed - but if, prior to upgrading to version 4, you wrote a record with the values %{id: "1", optional_string: ""}, that record will still be returned as %{id: 1, optional_string: nil}; if you write a record after upgrading, say %{id: "2", optional_string: ""}, that record will be returned exactly as written, %{id: "2", optional_string: ""}.
Prior to version 2.2.0, ex_aws_dynamo was encoding MapSet data into one of the three Dynamo set types (number, string, and binary), but was decoding those types into Elixir List data. In version 2.2.0, a PR added a boolean configuration option, :decode_sets, that would allow the user to specify that these types should be decoded into MapSet as well.
In this major release, Dynamo set types decode to Elixir MapSet by default, so the :decode_sets configuration option can be removed if it was included. If it this option was not included or was explicitly set to false, you'll need to be prepared to handle any decoded Dynamo sets as MapSet instead of List.
Several months ago, ex_aws switched its default JSON codec from Poison to Jason (see ex-aws/ex_aws#658). In this major release, the encoder used for dev and test environments has been changed to Jason, and the README file has been updated to show Jason in use instead of Poison, and refers the user to the ex_aws docs for information about how to provide a custom JSON codec.