First the AD you occasionally see means Auto Dry and is not an error code.
The E4A error is suppose to display when the drying time as been exceeded for the cycle chosen.
You say the error is displayed after only 20 minutes and this would be a little quick for this error.
The E4A error should not come up after only 20 minutes. Typically for most cycles the control would assume a time of approximately 40 minute or an hour. If the clothes don't dry then the control would shut things down and flash the error code after the estimated drying time has been exceeded.
If the dryer is not heating then that is most likely the cause of the E4A error.
On this dryer when it stops heating the most likely cause of the problem is an open thermistor.
If the dryer doesn't heat I'd suggest you replace the thermistor first and see if the problem is corrected. See the images below for more information on the thermistor.
If the dryer does heat but the E4A error keeps coming up then I suspect the control board is the problem and will have to be replaced.
I hope this information is helpful to you but submit another question with any additional details for further assistance.