V5R3 can't open V3 files. But, I opened some V4 files under V5R7.
You can try the higher version.
If you are urgent, I can open them for you or convert them to the IGES or other data file.
I have V5R6 installed, you are rite, i cant open the V4 files, i had try using EXP and Model files, both cannot even they contain complex surface or simple wire frame. But ProE is much more powerful and some times it work. I had use Binary mode for all files trnasfer but still fail. Any idea ?