As for the monsters themselves, most of the classic monsters planars will encounter have already been converted to 4e by Wizards. Even if the fluff doesn't work with Planescape anymore, the mechanics are sound and quite usable. There's a lot more out there that never made the jump from 2e to 3e, let alone 4e, but what's there is plenty usable.
Again, if you're really interested in doing conversion work, go see what they're doing at Planewalker. Better hurry, though, because they're well underway already.