Originally Posted by Lets travel
E4 means the defrost sensor is the problem, this is located up in the unit on the roof.
Actually, the defrost sensor is located IN the evaporator coils. It's also called a freeze sensor and is a metal cylinder about the thickness of a lead pencil, perhaps 2" long with IIRC a thin, white duplex wire coming from it . It is pressure fit between the fins at the front of the evaporator.
As they rarely go bad, I would suspect that either the plastic snap-in connector has come loose or it has lost contact with the upper relay control board.
You will have to remove the upper shroud along with the styrofoam cover over the evaporator section and then the plastic eval. housing to access the freeze sensor.
If you have non-ducted air, you might also be able to access it by dropping the air distribution box that's located on your ceiling.