The R2 and R4 are, as already mentioned, in reference to the cutting radius. I'ld like to expand on that to explain what is meant.
Imagine a the cutting edge of a 2 inch disc blade ... across the side of the disc, measure 1/2 an inch, and that is the arc that you present to your work surface on a 1/2" R2 cutter. An R4 edge would be a 4-inch radius disc, so the arc would be much less pronounced, or flatter. This does not change the size of the cutter itself, just the arc presented when you are using the tool. An R2 has more of a round profile, so it's easier to use to make gentle curves, as it is like the tip of a round nose scraper that has been ground to be much blunter. (Not a gouge, as there is no valley or inner curve to help guide wood down ... the flute, in other words.)
Now, the R4, being a flatter profile, would probably be preferable to penturners, as the curves we GENERALLY make are very shallow, if we want any at all, and getting a nice straight profile is easier with the flatter face to present.
If you are into doing detailed profiles, the R2 in the 3/8" would be the better choice, as it will get into tighter spaces and help with deeper profile cuts.
If you are into doing flatter cylinder shapes and fast roughing work, the flatter and wider profile of the R4 in the 1/2" would serve you better.
The other 2 choices are the "in betweens" ... but whichever cutter HOLDER you choose overall will dictate the size of the bits you purchase, between the 1/2" and 3/8". They are generally not interchangeable between bit sizes.