For that amount of lighting and that small of a space, I would get 2 computer fans and run them at a variable speed. You really dont need much cfm for the size box you have. Check it out:
You want to change the air in your box every 3 minutes if you can (for optimal growth) so:
your box is 2x2x4 = 16 cubic feet
16 cubic feet / 3 minutes = 5.3cfm fan
Most computer fans are around 30-80cfm so you should have no problem finding a cheap fan. Just make sure you dont get a VARIABLE fan. I did that and the computer has to change the speed, and by default its on the lowest speed