Not to disagree with Steve, but I think using the word "hackjob" is a little harsh when that final iteration of the RC4 (1/2″) is arguably one of the best shocks in the history of mountain biking, particularly from a reliability standpoint. The reason they kept the oversize reservoir and simply re-labeled the volume adjuster is because the shock was an end-of-the-line product: it wouldn't have been economically viable to redesign the parts for a shock that was in its final year of production. Despite this lack of further optimisation, the shock is lighter than virtually all competing products (BOS, CCDB, Vivid, and also lighter than the outgoing 5/8″ shaft version of itself). Functionally there's no real disadvantage to keeping the old form factor provided that IFP depth is adjusted to suit. The B/O adjuster can be removed to save a few more grams if desired since it no longer does much.
However - this shock (the 1/2″ / small shaft) is certainly the best of the lineup - because the fat shaft and BV cater for an era of frames that no longer exist (ones that require a pedalling platform and greater end-stroke support due to frame design). The circlip design on the piggyback also saw multiple iterations, and the small-shaft shock has the final version which involves a thicker lip on the DSC retention forging, and a single thick circlip instead of the multi-wrap design which had a tendency to pop itself out. The very last iteration of the 5/8″ shaft shocks did get the updated circlip but the 1/2″ shock guarantees it.
Thus, the final iteration is the best performing (in a modern frame) and most reliable shock of the lot in my experience. Unless you have an older frame that specifically warrants the use of the 5/8″ shaft version, then I think your original assumption is indeed the correct one - the 1/2″ shaft version is the one to get. It has a significantly lower breakway force than the 5/8″ shaft and in my experience is competitive with modern shocks (if not better - I run it by choice over the X2 and DB).