400 is better suited to a 2x3. 4x4 is approaching 1K territory.
Would Getting Another 400 Watt Outfit Be Worth It?
Myself I would look at better use of what you have before adding more heat to the mix. Seems that an area your size should be served well with a 400, if you have a decent reflective device to concentrate your light effectively. By adding more, you will be adding more light, but you will also be adding more heat to the mix. With fall and winter around the corner you might be able to have it work with little impact, but really you should be looking to gain control over your environment now before looking to increase your problems with additional lights.
Get the basics under control, then look at a 600 down the road if you really like. A 600 would give you more heat to remove, and the potential for higher yields, but it would also require you to have your environment nailed as to prevent any and all potential issues well in advance.
Good luck with what you go with. In the long run you will find out what works best, just takes a little hit and miss at first.