Also note that Serial Ports 3 and 4 share a single UART and therefore are useful pretty much only for 1-way serial communication if both ports are in use. If you require 2-way communication rely on Ports 1 and 2 and if you must resort to using Port 3 or 4 for 2 -way , ensure that the other port that shares the UART (3 or 4) is not used.