You shall not execute the file, but create it.
(later you may execute it).

But it's a good idea to test the command first:
Code:
ifconfig eth0 192.168.100.11 up
and if it works (no reply)
Code:
echo "ifconfig eth0 192.168.100.11 up" > /etc/rc4.d/S50ifup.sh
/etc/rc4.d/S50ifup.sh