The odd results are that you got 1.5 psi at 4 feet of water. Keep in mind that I run numbers for what they are and 4 foot of water should require 1.73 psi to pump water down that deep (0.433 psi per foot of water). Maybe your not in exactly 4 foot of water and pressure gages can certainly be off a bit.
The good news is that your plumbing is not restricting your air flow. So, less restrictive diffusers should put you in a good place. Matala and Vertex are common brands thrown around here at PB. Check out Hoosier Pond Pros for the Vertex brand when you start your search for diffusers. I've had the vertex diffusers in my pond now for about 5 years and got them for esshup (PB member). Here's a link if you want to check it out...
https://shop.hoosierpondpros.com/Aeration_c3.htm
