My company is finally upgrading away from a product that is written in a 4GL language. This product probably started out on a Unix but was ported to Windows decades ago. It has both a web and classic VB front ends.

All the source code is available and theoretically I could make changes and compile it up. The language itself is basically just plain procedural code but with SQL mixed right in -- somewhat like DBase or Foxpro but worse. I think the compiler produces C code and is then compiled with C compiler but it's been a while since I looked into it. Requires a version of Kornshell for Windows as well.