Hm. I know in one of my first dungeons, either the eladrin warlord or the minotaur fighter got killed after the group of 1st levellers pulled two encounters on them at once. Fun times!
I had an entire party killed when they fought "the beast of the black hills", a level 3 solo brute I made up that really shouldn't have destroyed them... but the party didn't work together.
Same players, different PCs also drew a 4 character TPK when they were investigating some ruins and were attacked by needlefang drakes (gah!) and some weird psychic wolves. Things did not end well.
A feytouched warlock was killed by halfling criminal minions. The fight was pretty simple - one halfling slinger, and about sixteen minions... on an open dock. The PCs ducked, except for the warlock, who targeted the slinger, and missed. And then drew sixteen ranged attacks. DEAD.
I think those are all the deaths we've had since 4e started, but I could be missing one or two. But were I to count? I'd say it was around eight or nine. NOt so many lately, though - my players have learned to work together.