I think you'll get better results just using STEP or IGES. Since V5 uses .CATPart and .CATProducts V4 .model files still have to be translated. I'm a V5 user and have experienced problems bringing .model files from V4 into V5. If there are problems from V4 to V5, the problems may get worse going from a ProE .prt to a V4 .model and then to V5 .CATPart.
FYI, if you are using SP1 or SP2 of V5R10 there are some problems with importing that are not in V5R10GA.