Rich, on both my RG's I can get a fourth (5th fret C) with proper setup, and MORE. No shim and my 550 goes to D (6th fret) quite easily.
My 520's trem sit's slightly lower with the action I like but will still give me about a 'sharp' D flat.
I prefer a perfect fourth, so I actually have a stopper in the trem cavities (plus I go a little nuts sometimes and with that kind of pull-up I'd break strings much quicker.)
I set my Edges up properly, no forward tilt, there are no shims under either neck (the 520 has the neck pocket angled so no shim is needed as I've seen on many other Ibanez). The 550, using the seam where the fret board meets the neck actually looks like the neck is lined up parallel with the top of the body, but a little higher than on my 520.
Now, having only set up one Jem, and never owning one, I can't say if the Jem has more or less, but the RG's rout does give quite a bit of room, judging by the two I own, if they are indicative of the others, I'd say a 5th can be done on most of them. Mine just happen to not need a shim (well, the 520 would, but not a large one) to get that kind of pull up. :-)