The most basic reason behind the 6/4 vs. 4/4 cap screws is due to the increased surface area of the forward ring which in turn gives you more clamping force/hold on your optic (especially useful for lighter rifles with heavier recoiling calibers). Most folks I see that run the 6/4 combo rings run them: a) because its OEM for certain mil-spec rifles (i.e. - the SIMRAD attachment point/ring cap that Rob01 previously mentioned); or b) purely for looks.
Seekins rings are so well made that there is more than ample clamping force just about any rifle/optic combo I have used with the 4/4 rings, even on extremely heavy recoiling rifles, but the 6/4 rings are just as excellent if you can't source the 4/4 rings in a pinch and they do "look" cool and give you just a little more juice when needed/wanted.