I think adding hose would reduce your pressure, but that may just be a good guess. My thought is that it offers more holes for the air to escape and that should relieve some pressure. I would consider adding 5 foot of straight right before the one you have already made and test it in the pond at the same 4 foot depth. If that drops the pressure to your satisfaction, you could reconfigure your diffuser/s to have the added 5 foot. Bump up the length to 10 foot if you get some drop with 5 foot added, but not much, and test again.

You could move the diffusers into 3 foot of water instead of 4 which should drop your pressure by 0.4 psi. Your pond should still get turned over with the diffusers in the 3 foot of water.

The best you can do with the diffusers in 4 foot of water would be about 2psi. I'd be shocked if you could add enough hose to achieve that, however. I'd be very happy with 3 to 3.5 psi and if it stays at 4 psi, it's good, but could be better.

The diaphragm membrane diffusers are definitely a favorite, but I'd hold off since you already have yours made. I have not used the soaker hose heads, but hear that they tend to clog after a while and do not clean up real easy. Watch your pressure gage...a rise in pressure would likely mean that they were clogging. I suspect that would take a season or two...thereabouts.

The membrane diffusers may require a bit more pressure to get them to start to produce bubbles...In the neighborhood of 0.5 to 1psi. I have never tested a soaker hose's pressure requirement. Should you take one of your diffusers out of the pond...note the gage reading while running and the head out of water...then remove the head and record the gage pressure. The difference between those numbers should be the back pressure from the head alone. If it is around 0.5 to 1 psi then the membrane diffuser should swap in it's place with little pressure difference.

Fish on!,
Noel