The four FM listed are the only "true" turn-based RPGs. I have a pretty damn loose interpretation of "RPG" that includes all the games everyone else listed as well as Loom.
Some of these games belong in other genres, but if you're an RPG lover, they'll satisfy you to some degree:
Beyond Shadowgate
Cosmic Fantasy 2 (too many battles)
Dragon Slayer
Dungeon Explorer 2
Dungeon Master
Dynastic Hero (more of an adventure game)
Exile (too easy)
Exile 2 (very hard)
Loom
Might and Magic 3
Shape Shifter (a lot like Cadash)
Ys 1-2
Ys 3
That's 13 games out of the 45 CDs, or 28% of the US releases that are Adventure/RPGs. Even if you only count the four turn-based RPGs, that's still 8% of the library and probably on par with, or better than, other consoles for its time. The TG-CD also has four mystery games (Sherlock 1+2, JB Harold, It Came from the Desert) and two strategy games (Vasteel, Lords ot Rising Sun) that may appeal to thoughtful, problem-solving gamers.
Now for a completely irrelevant rant...
Why the hell do games have to look like Chrono Trigger or take place in a medieval setting to be considered RPGs. Take the original Metal Gear on the NES for example--totally an RPG. Why doesn't anyone considered it an RPG? Because it was created when RPGs were barely even a genre and because it doesn't look like stinkin' Final Fantasy-cutesy-village-crap. Instead of a village you have rescued hostages and a radio; you collect keys (key cards), weapons, and healing potions (rations); explore dungeons (fortresses); and even gain levels (rank). Heck, the stealthy box might as well be an invisibility spell. It's an RPG, damnit.