Hey Smiler,
I was thinking about getting a fourth axis as well. I asked Keith (Kip) about what he recommended. I was also looking for a 4" table... anything bigger won't fit under my gantry.

He recommended the RDG table you posted a link to, and he also said something about knowing someone / having the appropriate stuff to do the stepper motor conversion for not very much.

Unfortunetly, because the stepper motor mounts on the top of the table it won't fit under my gantry, so I'm going for this one I think.