i run a 6'' air cooled 1000w hps in a 4.5x4.5x7.5 tent and i get some really nice buds.
The tent gets about 8 to 10 degrees hotter then the intake air temps,so my temps are between 78 and 85 through the summer, alot more stable in the winter.
i think 1800 would be too much for a 4x4x7 tent