Not meaning to beat up on Ragnar, he's usually helpful and reliable with his tips - however, in this case it's slightly inaccurate.
1) A V4a comes with a Rise chip, split-voltage planes, and no SW4 (fixed 2.8v Vcore).
2) A WinChip from a V3 (or earlier) will also work in a V4a/b or V5 with no mods. This little bit of magic happens to work because a WinChip was designed to be *both* Socket-5 and -7 compatible, i.e. its Vcore can range from 2.8v to 3.45v. It's the only Pentium -class chip with this ability to work in both single-plane and split-plane boards.
3) A Rise MP6 from a V4 will however not work in a V1, V2 or V3 (with a single-voltage plane), unless the split-plane mod has been performed (search the postings for info on this mod).
4) Generally, the split-plane (V4 and later) units are more versatile in CPU choices. You can mod the V1/V2/V3 to obtain the same versatility as the V4/V5.