If you buy 400hrs, is that for 1 account? If you run 4 accounts for 100hrs, does that mean u lose 400hrs?
You can run up to 4 accounts at the same time. You can use as many RuneScape accounts as you like. The hour limit doesn't care about how many accounts you run - you could run for 100 hours on one account, 250 on another, and 50 on another and that would make your 400 per month.