Im thinkin with 600w HPS youd get a great yeild with no floros needed. But i think a 400w HPS youd still get a great yeild and it might help a bit to add a floro or two if youve got the room. Im kinda in the same situation, cept i only have 3 plants. Im in a space about the same size with a 400w HPS and one floro directly above my clone [1/2 way up on the other 2 plants] and it works great for me. I guess it all depends on how big you wana veg your ladies.