The R4 and R2 indicate the radius of the cutting edge. R4 means a 4 inch radius and R2 a 2 inch radius.
As far as 3/8 versus 1/2 the is the width of the cutting edge. If it is a R4 or R2 or just straight square cutter the 1/2 would be wider across the cutting edge. Either one of those width will work fine for pen turning. I have a set of 3/8 which came with the diamond, square and round cutters; and recently was able to get an R2 to complete the set.
Hopefully this information helps. If I miss spoke on any of this information hopefully more experienced turners will set us both straight.
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.