600W HPS is fine for a 4x4 but as you said heat could be a issue with HPS especially in summer, i you can afford some LED you should look into this:

4X4 - Spider Farmer Official (spider-farmer.com)

this are the model from spiderfarmer that would do a great job with a 4x4 but there some equivalent with some other brand, like viparspectra, phlizon, mars hydro etc.