Thank You! My tank has the same dimensions as yours. The choice for 8 XR30's was 2 fold, I wanted to try as best as possible to distribute par evenly throughout the tank and eliminate light hot spots. My lights are 9" off the surface and roughly 19" apart on center (8" between each unit). Having 8 allows the lights to interact with each other giving more consistent par levels and will allow the lights to run at a lower overall intensity and hopefully prolong life expectancy.
With that being said, with every channel at max and overall intensity at 100%, the par levels of these Gen4 Pro's are off the charts. I have no doubt in my mind that 4 of these would be plenty for an SPS tank. Although when considering light distribution and Ecotech's recommendations of one XR30 covering a 24"x30" footprint at 9" off the water , you're right at the edge of their recommendations and may have low light areas in between each unit. I personally would still orient them lengthwise, although, you may consider mounting them turned 90 degrees front to back as well as raising them up to 11" to get better color distribution as well as even par.
Are you on the fence about wanting to get more? or is 4 your limit?