i would go with the 4x4 tent because you will want the extra space after a few grows, the cost depends on you electric cost per kilowatt, mine is 13pence so for every 2 and a half hours the 400 is on it cost about 13p, so about 94p a day for 18 hours on, 12 hours is about 65p, you can run a 600 for the same price 12/12 as the 400 costs for 18 hours, so taken that into account if you veg with the 400 when you switch to flower you can put the 600 on for no extra cost.