GSM2 Click Interfacing.

It could well be the newlines that are causing problems. As @nicolasc_eu says, try using Serial.print and explicitly sending the newline character that you want.

Just a note that if you want to send 0x1A, you can do:

Serial4.write(0x1A);
Serial4.print("\x1A")
Serial4.print(myOtherData + "\x1A")

but the following won't work:

Serial4.print(0x1A);
Serial4.print("0x1A");