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You could instead run something like a PAC LC-1 up front, inline with your SUB RCA cables to control all sub amps input level (depending on your setup of course). I would be more inclined to go that route, it should work fine with 4 amps, and controls the level instead of bass boost. The RF bass knob controls the bass boost level, which may or may not be what you want.
Rockford use to make a splitter with a master control knob, could control up to like 8 of those BD amps with it. I had one years ago, but sold it. Somewhat hard to find item though and can be pricey. Was a black box with 8 ports you mounted in the rear of the vehicle which you then plugged your amp(s) remote port into, then a dual wire remote mounted up front, not much info on those units, but they made them just for this task.
In fact, here is one on ebay:
You could try a basic splitter on your 4 amps, I'm guessing that is essentially what the RF box did, but I can't say for sure and I no longer have mine to see exact function.