4.2 amps x ~55 volts is ~230 watts. I wouldn't run more than 200 watts passive. adding a fan blowing on each heatsink and you'll be fine. Yes, constant voltage to get that current level. the HLG-480h- 54A would probably work for 2. Personally I think 2 on a meanwell HLG-320h-C2800 is about perfect.