According to https://www.iana.org/assignments/service-names-port-numbers/... the lowest unused ports are currently 4, 6, 8, and 10. I think these were originally the port numbers for the opposite direction of data transmission for services on ports 5, 7, 9, and 11 (RJE, echo, discard, and systat) but that function became obsolete with the switch from NCP to TCP in 01983. I may be misunderstanding this a bit because I don't really understand NCP.
But let us know.