If you are leveling 4 groups, assign key "4" so that only members from group 4 attack with instant spell/or your single target dps macro, to tag the questmob(s). Then carry on towards group 1 with key 3.....2....and 1... and you are done.

Keep in mind that sometimes, after you complete a quest, that group can get phazed. So its good practice to complete all the others "4/5 complete", then when all groups have 1 more kill from completing, start from the last group and go towards your main group.

Whenever i levelled only two groups, i used 1 mousebutton to have group 2 tag and 1 button for group 1. Worked nicely.