Radions are rated for a 24"x24" area so you would need two per tank if it was a standard 4x2 footer, not a cube. I have 4 over a 7' tank but it's only 24" front to back and the lights are hanging horizontally which gives me a little more spread and makes me not believe the 24x24. I just took some photos that show the spread of one light hanging both ways and there is definitely a difference. If my tank was more like 30" front to back and no coast to coast overflow, i would hang the lights the standard way and would probably need five.
I'd almost say you would want 4 over that tank. Two would work over a 24-36" cube if you get creative with coral placement on the larger. Odd numbers are hard to position over a cube.
The extra color channels of the pro give your tank that extra pop.
