I imagine you're asking only about pre-built fixtures, correct? If I were setting up a 4x4, and I were choosing a pre-built, there's no question: HLG-600. Cheaper than the 550, and a little more headroom (for those especially robust strains that can take a LOT of light).
However, if I were truly setting up a 4x4 now, I'd go DIY, and for DIY, nothing beats strips for the cost. Samsung F Series Gen 3 is the current King of the Hill.
That said, I fucking love my Quantum Boards - perfect for those who want to DIY, but aren't into the amount of work that goes along with strips. The light in my avatar is a 4xQB120 (60'ish watt 120 diode Quantum Board); runs up to 250w, but I have it in a 2.5x2.5, so I typically don't run it higher than 200w. I actually built two identical fixtures (in identical tents). Spent less than $1/watt on these things, and they'll be growing my meds for many years to come, without having to think about changing bulbs or reflectors, or any of that mess.
Edit: actually, if I had to buy rather than build, I might opt for two 260w lights from HLG instead of the HLG-600. I guess it would depend; I like the simplicity of a single fixture, but the versatility of two would also be attractive. But basically, either option would kick ass.