Gene,
As mdorantes stated, the latest is OS4.0c.
Since the jump from 3.x to 4.x the sounds improved very much : e.g. cleaner epiano's.

Since 4.x problems as note-dropouts and latency while playing full chords were introduced. Fortunately Ketron improved the OS recently and they reduced the problems to almost zero.

I would go for 4.0c with new sounds if I were you.

I do hope Ketron finds time soon to eliminate ALL problems.