Honestly, just tell her its a hobby of yours. I must say 4 nights a week is a lot just for raiding though.
If she doesn't accept, ask her if she'd think the same way if it was for football/soccer (w/e country u're from u'll understand haha). Those people spend 2 nights a week training, and a whole day for a game so yea. A hobby is a hobby, wether its football or wow.