a 4mm setup will work better with 33 - 39 mm carbs. stock will work but you are not gonna be gettin the full potential of the motor.
you could get by with either, but do you ride faster or slower most of the time? play riding sounds more to me like dune or woods port would be better.
i dont know shit about RB but i know jeff is good and also redline racing in oregon. i highly recommend both and have motors from both.
you riding fast most of the time? if so cubs would be fine. however, if you ride trails and shit i would go with stock cylinders and the right porting for your riding.
a cub comes in any size and setup correctly will make more power than any stock cylinder setup of the same size.
no problem, thats what we are here for.