Just play through them normally.
Seriously, combat encounters in 4e don't that as long as people paint them to take in 4e. Of course, it depends on DM/player posting rate, but if they have a good posting rate (1/day at least), combat hardly take more than a week to finish - and I'm talking about major combats, easy or simple encounter usually last only three or so days. Of course, if the players are slower to post, it will take longer to finish the encounter.
A method I've seen used is halving hit points for all player characters and monsters, and halving healing effects as well. It makes combat faster, but also more deadly, so this should be used with caution.
There are other, non-mechanical ways to make combats end sooner as well. For example, if the encounter is clearly a victory to the PCs but there are still monsters standing, I'll just write them dying, running away or surrendering. There is no need to extend the combat for another round or two when there is clearly no threat any more.
I hope that helps!
Edit: Oh yes, also one major factor in speed is DM organization. If you have your stuff straightened out - monster tactics, updated maps, status tables, or whatever you feel more comfortable doing -, then you'll be checking results and making updates much faster, which will in turn allow players to post faster as well, if they are on-line.