Alarm 4076 on Fanuc resonators means that the power measured by the power sensor is way below the actual power commanded (that max power decrease value varies depending on the resonator power; could go from 500 to 1000W).
The problem could be as simple as a bad power sensor. You can measure the resistance of the sensor; should be around 250 Ohms.
It could also be a bad laser gas (maybe try a new bottle, making sure the specs are correct for the type of resonator you have; they are not all the same).
Could be an internal leak. Do an internal leak test. Do also an external leak check.
Or the optics could be dirty or misaligned (mode issue). How many hours do you have since last optics replacement?
Usually, if a power sypply or matching box is bad, you would get a 4063 alarm.