Four skirmishers would probably take about 2-4 rounds unless there's some crazy crits going on that take them out faster. Up to 6 rounds if everyone rolls like poo.
It might be a bit of a boring fight having 4 of the same creature, though. I think adding in a slinger or two or something like that might make it more fun. Or, if you just want the 4 skirmishers, definitely create terrain that is advantageous to the skirmishers. Otherwise, the PCs might steamroll them. 4e works best when using rooms with lots of features and a good mix of enemies.