Now you see why I would have opted for using multiple
lights points for a more even coverage, rather than a single light source. But still a 600 watt light should be plenty for the small 4x4 foot space your trying to cover. If raising the light up to cover the area better works, you shouldn't have any problem with light intensity (lumen drop) with that 600 watt bulb as long as you don't go beyond 4 feet. But if you can rotate it for better coverage and get it at 3 feet, that would be even better.
Direct heat from the lamp to the reservoir can be a problem in itself like you mentioned, but the reservoir temps will usually be close to the ambient air temperature even if you block heat from the lamp. In other words if the light is raising the ambient air temperature in the room, it can indirectly raise the reservoir temperature through the air temperature as well.