I’ve tried pretty much every way you can think of when changing the LP filters.
IMO what you should have done was 1st open the bleeder screw, then try your way of cracking open the fuel filter then run the LP until fuel starts coming out.
Go up to the bleeder screw, run your LP when fuel starts coming out close the bleeder screw.
You could also run the LP for a while without starting the engine, and without opening the bleeder screw. air should return back to the tank.
This takes several minutes.
What I do is get my fuel down below 1/3 tank, drive the front on blocks, loosen the fuel cap, this way I don’t have any fuel running out other than what’s in the filters.
Change both filters filling with fresh fuel, I still have the stock fuel filter, cycle the LP on and off several times observing the LP’s return flow, after several cycles you’ll notice less air bubbles.
Usually I won’t have to loosen the bleed screw, but if needed or feel in the mood, I’ll turn on the LP, crack open the bleed screw until no more air.
Then start your engines
The cp4 should not even be considered a problem changing filters without the engine running it doesn’t play a role.
The LP should be the only motor running when trying to fill the filters and bleed air out of the fuel system.