I am trying to update my software on my iPad and before it will download it has asked for a 4digit pass ode. I've never had one
Your iPad certainly thinks that it has a passcode.
If you can't remember it then you will need to try to restore your iDevice from the same computer that you most recently synced to.
Read this: http://support.apple.com/kb/ht1212
If you can't do the restore then you will need to try Recovery Mode.
Read this: http://support.apple.com/kb/ht1808
Follow the instructions very carefully.
The passcode is the four digit number that you enter after an Autolock on your device or when you first turn it on. If you can't remember that code you'll have to restore your iPad before you can do the update.