rcS: initialize TELEM4 on ttyS3 on FMUv5

ROMFS/px4fmu_common/init.d/rcS

@@ -552,6 +552,12 @@ then
 			mavlink start -r 20000 -b 921600 -d /dev/ttyS0
 		fi
 
+		# Use ttyS3 (TELEM4) for MAVLink on FMUv5 in addition to ttyS1 (TELEM1) and ttyS2 (TELEM2)
+		if ver hwcmp PX4FMU_V5
+		then
+			mavlink start -r 2000 -b 57600 -d /dev/ttyS3
+		fi
+
 		if ver hwcmp AEROFC_V1
 		then
 			set MAVLINK_F "-r 1200 -d /dev/ttyS3"