No worries, I'm glad you posted in open Car category since it is a Mitsubishi radio (there is no Mitsubishi section). I would not have seen it in the Nissan category, and it really belongs here anyway :)
Each preset 1-4 represents digits 1 through 4 of the code.
Press preset one repeatedly to scroll through the first digit until a 4 is displayed.
Press preset two repeatedly to scroll through the second digit until a 9 is displayed
and so forth with preset 3 and 4.
Once the correct code is displayed, you must push the unlock button, which varies but is always either CD, AMFM or TAPE. There is no penalty for pushing the wrong button as long as the code is displayed correctly. Once pushed the radio will beep once and turn on.
_IF_ you make a mistake, it will produce an error (typicallly Err1, Err etc). If you error 3 times, the radio will turn to "oFF". If this happens you must leave the key on in the ignition for about 75 minutes with the display reading "oFF" after which time the display will go dark, and the next time you turn it on you will be able to enter a code again.
I will be stepping out for about a half hour in just a few minutes, if you'd like to take care of the code right now, I hang around for a few minutes, otherwise I will be back in 30-60 minutes.