That I can't tell you, but can tell you that the pump will only see about 2 psi or a little over at the 4' depth.
The rocking piston compressors are a little overkill for that application as they are able to create higher pressures for greater depths. That one at 50 psi could pump air down to about 25' depth and would not loose a lot of efficiency doing it.
The pressure only rises to the point of overcoming the static water pressure at the depth of operation. Slightly less than .5 psi per foot of depth. Then any additional back pressure friction in the hose (minimal) or anything additional like a spring loaded check valve might add or the back pressure the diffuser creates (minimal on a membrane diffuser).
