[Myka]

Let's talk about that pump first. That Fluval SP2 is 950 gph at 0 foot head height. At 4'6" head height (which I assume is approximately the highest point of your plumbing) the pump will only deliver 290 gph (assuming I'm reading their weird flow chart with tiny writing properly), plus the more plumbing you add (tees, elbows, etc) the more the head loss will be affected. Return pumps don't usually have that much head loss. This is a fairly weak pump! So if you have a 90 gallon tank, and 20 gallon sump that will be about half full, you're looking at about 100 gallon total system volume. So this pump will deliver only 2.9x turnover not including the piping, so probably closer to 2.5x. Imo, that's really pushing the lower limit of what your return pump should be. If you can return that pump and buy a new one, I'd probably recommend that. The SP4 will be too big.

I'd suggest either a Quiet One 3000 (not as quiet as the name suggetss) for your tank which would deliver about 580 gph minus piping, so about 550 gph or 5.5x turnover which is right about perfect imo, or an Eheim Compact 5000 (quieter) which would deliver right around 600 gph minus piping.

Now, to the question about the outlets. For a 4' tank, I prefer to keep it simple and just do a single line with a single outlet. 500-600 gph is not a lot of flow, and splitting it into two and you could probably pee harder. Those SCWD are great on closed loop pump, but not return pumps imo. The little Hydor gadget is good for AIO nano tanks like BioCubes, but imo it's not that useful.

Use powerheads in the tank for waves. That said, most of the powerheads that will do varied speed are pricier powerheads. For your tank size, a couple Tunze 6055 or one Tunze 6105 plus a controller, or one EcoTech MP40 (built-in controller). Some people buy Jebao powerheads - they are very cheap and come with a controller, but if you buy one, buy a spare because it likely won't last 2 years. Or, just don't worry about waves for now, and buy the cheaper, not controllable powerheads. You'll need more than just the one you have though. Look for about 2500-4000 gph total combined flow between 1 or 2 powerheads.