Well, issues w/ light distribution and airflow...

If I were running a whole bunch of cuttings, I'd probabley go w/ the 4x4, because I'd know that I had plenty of overhead room for stuff like carbon filters, oscillating fans, etc. I'd be able to cram all that crap up over the reflector and still know that I'd be able to keep the reflector high enough over the plants to keep the distribution pattern sort of even-ish.

Doing smaller #'s of larger plants- you're gonna get tight on room by the time you hit flower set. Going to the 5x5 does cost you some intensity, but it buys you a whole bunch of room, allowing a certain amount of relected sidelight off the walls.

Also, you have to balance things on a total room yeild basis.

Let's say (purely for conjecture) that intensity and yield per square foot are directly ( 1 to 1) correlated. Let's also say, ( and I am making these #s up) that your cultivar of choice will produce 1 oz dry per square foot @ 9,062 Lumens/ square foot. Your 4x4 will produce a lb. ( 16sq'x1oz)

At 7,250 Lumens/ square foot, ( 80% of above) each square foot will produce .8 oz, assuming a 1 to 1 relationship. Your 4x5 will produce a pound (20 sq'x.8oz)

In the 5x5, the plants are receiving 65% the intensity of the 4x4. If we follow our admittedly-all-made-up formula, each sq' will produce .64 oz. Your 5x5 will produce , oddly enough, a pound. ( 25sq' x.64oz).

I love linear relationships. They're so...symmetrical...

So a lot of it comes down to personal management issues, grow style, cultivar chosen, etc. It's a lot easier to manage environmentals in 200 cubic foot room (5x5x8) than it is in a 128 cf room(4x4x8). Larger spaces are more tolerant of mistakes, easier to work in, etc.

It sounds like you're not that familiar w/ the genetics that you'll be running. I'd definately go for the larger space- It really bites to realize that you timed things just right to peg the room at 70% stretch, but the plants are gonna do 140%.

I like a k per 4x4 if you're running say, 4 of them in an 8x8 room, but cramming one into a 4x4 presents management issues. Make it easy on yourself- go a little big.