Assuming compilers and runners are from same versions, can they be executed on any platform (i.e. UNIX, LINUX, WINDOWS, DOS, etc.) without the need to re-compile the source code?
DOS - no; there are no common versions for DOS and the other platforms.
AFAIK, the .frm and .arc files are platform independent.
For the most part, the .4go and .4gi files are platform independent. I seem to remember there being an issue about 15 years ago with 32-bit vs 64-bit p-code structures, which was basically a bug. However, that may have been more of a bug on the DEC Alpha than a generic 64-bit I4GL-RDS problem.