First of all checkout Growmau5 DIY COB video series on youtube, its in his playlist.
4 x 2 = 8 SQ/f youll want 900 PPFD.
I would checout @CobKits and the Citi CLU Series.
Building the light is simple.
If you want some help PM.
Dont buy a chinese light, 1 your not going to get the wattage thats...