Turn ignition on, turn radio off.
Hold the as button on the radio and turn it on while holding it.
Hopefully the set will beep and show ---- instead of safe.
Let go of as button, enter the code by pressing number 1 repeatedly to get the first digit up. Press 2 repeatedly to get second digit up, likewise 3 for third number and 4 for fourth.
The set should now be showing the code you want, when it does press and hold the as button again until the set jumps into life.