Every AC manufacturer has its own set of error codes. So, when there is a problem with your AC unit, it shows such error codes like E3, E4, etc.
These error codes display particular issues that the user or the technician can use to determine the exact problem with the unit.
If your AC unit shows such error codes, you can go through the operating manual that comes with the unit to understand the issue. Otherwise, you should consult with an HVAC technician.