4e and the original Tomb of Horrors isn't likely to work very well.
I've used 4e to redo some old modules, particularly G1 (Hill Giant Steading) and it can work fairly well. With a little experience you can choose the correct monsters and (de)level them as necessary.
Two areas that I'd be very concerned with when trying ToH in 4e would be the presumed lethality of ToH and that AD&D typically tests players, not characters.
Save or Die
Before 4th edition, D&D had many save or die mechanics. Tomb of Horrors takes this one step farther and has several mistake and die encounters. Stick your hand in the wrong mouth, or don't respond to a tipping hallway immediately and your character will suffer severe consequences.
4th edition does not have any real way to model this. 4e players are not used to having their characters die without recourse because of one decision (that they may have made minutes ago even). You could introduce these, but it would be a big paradigm shift. In AD&D you could have a new character up and ready to rejoin the adventure in a couple minutes, with 4th edition, you're doing good if you can get a new character ready in under an hour. I'd argue that just that one change means that characters shouldn't die very often. It sucks to spend half the session creating characters because you died a few times.
For me, the Tomb of Horrors without frequent character death/disability just wouldn't ring true.
AD&D has players, D&D4e has characters
When playing AD&D it is expected that players will ask questions about and interact cautiously with the environment. It can take a great deal of time for an experienced party to decide to walk down a seemingly empty corridor if they have decided (rightfully or not) that the area is dangerous. They'll break out the 10' poles, maybe some marbles, perhaps some flour to look for invisible critters...etc.
A 4e player will have his character make a perception check. 41! She'll proudly announce, what'd I find? If the answer is "nothing", the party will march forward.
It's a completely different vibe. The Tomb of Horrors requires very cautious experienced players. Even with extreme caution, it is likely that most characters will die as there are several places where the correct answer cannot be divined in advance.
A solution still exists!
What you can do, is run your current 4e characters through the Tomb of Horrors in AD&D using avatars of themselves. As long as the players are clear that these characters are themselves, yet different, it can work out and be enjoyable for all.
Things that my group has done
- Play 30th level avatars battling Orcus
- Shrink and go Through the Looking Glass (used many munchkin rules)
- Visit an alternate reality and become vampiric versions of themselves
It's up to you whether dying as an avatar has consequences in the "real" game. It can work both ways depending on how you set it up. Keep in mind that outside of blatent DM fudging, a TPK is by far the most common result of going into the Tomb of Horrors!
You can either create the AD&D avatars yourself if you wish to surprise the group or engage them in the process. It would also give you a good chance to push out some notes on the differences between AD&D and 4e if your folks aren't already aware of them.