H.A.F.
a.k.a. Rusty Nails
Grow for your lights, not your tent. If you can efficiently light 4x4 of your 5x5, center fewer plants in that area - accept the fact that either the outside plants won't do as well (rotate?) or that you raise the lights a bit so everything does equally less awesome.I CAN get by without another light, however, to FILL the 5x4 space, I'll need to invest in another light.
If you think about it, the outer edge is fan leaves - all the way around. the tops in a 5x5 are really only filling 4x4-ish, so there's an eqillibrium there somewhere. That's what I have been shooting for.