Tested today in Eye of the Storm - Win a game with 3 flags and stand on the 4th (I stayed there as a druid and unstealthed before the game ended). Once the game has ended, stay in the match and watch the meter tick over. Once it does, you'll get credit towards the 4-cap achievement.
Not sure if it's doable with 2 people in a 2-cap win, etc. Will test and report back!
