srcgen4.csproj

<!-- This goes in the top-level property group -->
<PropertyGroup>
  <LangVersion>preview</LangVersion>
</PropertyGroup>

<!-- Add this as a new ItemGroup, replacing paths and names appropriately -->
<ItemGroup>
    <!-- Note that this is not a "normal" ProjectReference.
         It needs the additional 'OutputItemType' and 'ReferenceOutputAssmbly' attributes. -->
    <ProjectReference Include="path-to-sourcegenerator-project.csproj" 
                      OutputItemType="Analyzer"
                      ReferenceOutputAssembly="false" />
</ItemGroup>

Change the include to path-to-source-generator. It's misspelled.

Thanks!