Test against latest version of ocaml 4

Author: andersfugmann

.github/workflows/workflow.yml

@@ -14,10 +14,13 @@ jobs:
           - ubuntu-latest
         ocaml-compiler:
           - 4.08.x
+          - 4
           - 5
         include:
           - ocaml-compiler: 4.08.x
             packages: 'ppx_protocol_conv.opam ppx_protocol_conv_json.opam ppx_protocol_conv_msgpack.opam ppx_protocol_conv_xmlm.opam ppx_protocol_conv_yaml.opam ppx_protocol_conv_xml_light.opam'
+          - ocaml-compiler: 4
+            packages: 'ppx_protocol_conv.opam ppx_protocol_conv_json.opam ppx_protocol_conv_msgpack.opam ppx_protocol_conv_xmlm.opam ppx_protocol_conv_yaml.opam ppx_protocol_conv_xml_light.opam'
           - ocaml-compiler: 5
             packages: 'ppx_protocol_conv.opam ppx_protocol_conv_json.opam ppx_protocol_conv_msgpack.opam ppx_protocol_conv_xmlm.opam ppx_protocol_conv_yaml.opam'
     runs-on: ${{ matrix.os }}