1 kW is 3413 BTUs so 1 watt needs 3.413 btus.....so ....600x4=2400 x 3.413= 8191.2 BTU's. That is assuming the ballast and bulb are in the room. So that is your MAX number of BTU's you should need. Try to figure out what proportion of heat is generated by your ballast and take that from 8191.2. That should be your number. Sorry I cant give you anything exact but I have no idea how much of the 600 watts is used for heating the ballast itself. But if you go with an 8000btu AC you should almost certainly be good. happy growing.