The vito's 3mm stroker crank has a slightly smaller big rod end than stock so the cases don't have to be machined for the crank to work. The only problem is on SOME cases the very top point (the very outlet) of the crankcase area has to be filed down. One side of my cases needed it and the other did not... so it's not universal.
Witht a 4mm stroker, hotrods used a standard con rod but set the crank pin farther away from the centerline of the crank... so the crankcase has to be machined out so the rod can pass around inside it.
You may get an extra 4cc's from the +4 over the +3. Once the cylinder is ported for the stroked crank, the difference is miniscule. If you are going to drag engine, the smaller big end of the +3 might make a difference.
If you're building a trail bike, the ease of installation on the vito's is probably worth the sacrifice of a few cc's.
I have the vito's +3 crank in my bike now. I have a +4 in the mail for a different engine..