For a 4x4 area...assuming youll have about 16 plants in there...youll need 16 each new cycle...because you're doing the sea of green method, it might be difficult to set up a perpetual harvest (rotating veg plants in, harvesting flowered ones).
Anyhow, youll need half the space, assuming that your veg containers are half the size of your flowering containers (transplant when starting flowering).
If you have a few moms in there, maybe a bit bigger 2x4 or so.
T5's dont compare to Metal Halide in terms of Light output, and growth rate...But T5's are cheaper, cheaper to operate, more reliable, and produce less heat. (Keep in mind, that your clones and mothers will grow very rapidly and large with the MH.)
A good veg setup can be had with just T5's...so unless you're looking to really spend alot of $, get T5's.