so a 4x4 is 16 square feet,multiply 16x40watts per square feet to get 640 watts.I would drive 12 cobs in a 4x4 and drive them soft,You can get the citizen 1212's for around 9 dollars a piece from cobkits.com.run them on 3 185 1400's at 4 cobs per driver at 197watts per driver giving you the 600 total watts you need,or 2 320 1400's for the same wattage.the driver wont change the watts.the drivers are like grocery carts just carrying your voltages.the cobs all added up multiplied by the amps is what gives you the wattage.
you could also drive 6 cobs if you wanted to save money on cost in a 4x4 by using the citizen 50 V 1818 which is a higher forward voltage.
using the
HLG 320 1050 you would have 6 cobs X 50 volts = 300 volts X 1.05 amps on the 1050 giving you 626 watts with 6 cobs instead of 12 but the tradeoff is they will run a little warmer being driven harder.