I think you can do horror-themed adventures in 4E rather well.
I didn't enjoy the Ravenloft *setting*, but I enjoyed some of the adventures once you put them in the 'real' world.
The recent Encounters series - Dark Legacy of Evard - was very true to the spirit of the Ravenloft horror: a town slips into the Shadowfell every night, and is attacked by dark denizens. A ghost possesses a wizard and must be hunted down and returned to its resting place. All this without actually needing "Ravenloft the Domains" to exist.
(Dark Legacy of Evard would have been far more effective if played over three nights rather than 1 encounter/week, I believe, but the concept was good).
Cheers!