The lumens that your HID bulbs are rated at are rated at 1FT above the canopy. A 600W HID will put off a fair amount more heat than a 400 or 430. You can get a 430 much closer to the plant tops without burning or bleaching. So if you have a 400W bulb rated at 55,000 lumens that is 1FT from the top of the canopy you are getting 55,000 lumens to the tops. If you have a 600W rated at 88,000 lumens and you have to keep it 2FT from the tops to prevent bleaching or burning you are only getting 44,000 lumens to the canopy. I run an air-cooled 430 over a 3x3 ebb and flow table in a 4x4 tent and get good yields, but I also keep the light about 8-12 inches from the tops.
If you are looking for more lumens then a 1000W will light up that 4x4 perfect, but you may run into heat problems. You will need at least a 200CFM blower just to cool the light. I wouldn't get a 600, either go big, or just keep the 400 and move it closer to the tops. Hope this helps.