What causes a waterheater popoff valve to leak I've replaced 4 this year

Are you talking about The T&P Valve see the image below?

If that is what you mean then you need to find the cause why it pops.

In normal operation of the T&P valve, no water should be discharged from the valve.

A T&P valve that discharges is an indication of an abnormal condition.The causes of discharge can be thermal expansion, excess system pressure, low temperature relief (T&P) failure, too high temperature setting, or something in the water heater causing excess temperatures.

You need to check the water Temperature and the water pressure, and let us know, Thanks.

John

If this has happened with the frequency you say, check the water pressure at your HW heater with a Watts test gauge. It should not exceed 75#. There was a post last week with a same question and he found 125#. If your water line is equipped with a backflow checkvalve and you do not have an expansion tank, you may also be overpressuring the system that way or you may have a bad main pressure control valve.