I'll be the monkey wrench.
Even with the horrible constriction of the pump's output by the 1 1/2"' feed line/spray bar the pump will still pump more water than your little filter can handle.
Start cuttin Ethan.
First, if you're going to do an upgrade you might as well maximize your new equipment.
The pump WILL NOT pump 4200gph if you make it draw through a 1 1/2" suction line. The pump WILL NOT pump 4200 gph if you try to squirt your water through 1 1/2" pipe either. You will increase pressure. You won't flow 4200. WAG? around 2400. You coulda bought a much smaller pump to get the same results unless you...use bigger pipe, at least 2", go for 3, on the output side. Try to bring your 4" feed all the way to the pump.
Second, you can improve your old equipment.
Hole saw and jig saw/sawzall maybe? Make a new spray bar. Remove the original and drill a new hole. Be creative when it comes to using a hole saw with nothing to hold the centering bit. The actual spray bar is easy, even if it takes you a couple tries. Get out your sawzall and cut open the waterfall end, maybe making two rectangular holes leaving a center piece of the original plastic.
Third, don't throw out the baby with the bathwater.
Your waterfall filter has a rudimentary provision to drain off settled waste from below the media. If you can keep this a functioning mode by keeping your new medai elevated from the plastic bottom you will be ahead in your efforts to create a user freindly system. Do daily maintanence, pull valves to waste, and weekly cleaning, backflushing and water change.
Fourth, figure out a method to supply water safely.
I don't remember reading if you were on a well. If you're using city water you will probably need a better procedure for treating incoming water. If you're using a well, you can rig up a Hudson valve, from Tractor Supply, with disconnects for the hose, from Tractor Supply, and keep the hose hooked up, mainting water level, except when you need to use it somewhere else. Big load off when you have to go help out as Family Guy. This saves a lot of trouble.
Take care.