There shouldn’t be any difference physically. An STL is composed completely of triangles (facets), and it doesn’t change from one computer or OS to the next. What you are probably seeing is that the display is different, there may be more visual smoothing of the facets going on with V5 Mac than in V4 Windows.
Try this on both computers: Maximize Perspective view. In the Viewport menu, check “Flat Shade”. That will show you the STL exactly as it is with all the facets, because it removes the visual smoothing from the display. It should then look more or less the same on both (well, the Mac does have a nicer screen than most PC’s, but that’s just hardware.)