Is it possible to read & work on InDesign files (CS4) from the PC (hard disc has been removed & cased as an external drive) if I now buy a Mac & purchase CS6?
As Bob says...
As far as working with the files, other than font issues if non-Opentype fonts were used there should, in theory, be no problems, and even font issues are realtively easy to deal with going from PC to Mac (but NOT vice versa).
I said in theory because we see occasional reports of files that fail in small wayys, or catastrophically after editing when directly converting legacy files by opening in CS5 and later. I recommend export the files to .inx or .idml from the original version and opeing that instead, but in this case that is not going to be possible, I suspect, so the nect best thing is to openthe files in CS6 and immediately export to .idml and save as new files. DON'T overwrite the originals. See Remove minor corruption by exporting