Each light (400 W MH) draw lots of current. You really wanted a heavy duty timer that take lots of current or it will fail quickly. You also need your light on different outlet, different circuit really so that you won't trip your fuse. I have a large tank with three circuits dedicated for my tank only (four id I count my 220 V circuit for my chiller. I have three pumps one on each circuit and 6 lights. I devided these light and pumps out so that one or even two circuit trip, it won't crash my tank.
I would have one timer for each light. You can get a 15 amp timer at HD for about 20.00.
Minh