Evap coils frost up for about the first 4 inches

Your mostlikely low on freon. You have a leak in system that has to be repaired. Its a big buck repair.
Other possibility is a bad compressor or a restricition in the sealed system. Also a big repair. If frig is under 5 yrs sealed system should be under warrenty.

This could be a low capacity compressor issue (It can pump the coolant down far enough to make it cold again. Or perhaps your coils in the back or under the machine are dirty and not able to dissipate heat. The last thing is that it could be low on coolant. If this is your main fridge, call a tech to take pressure readings to see if the coolant is too high or too low. Clean the coils in the meantime and see if anything changes.