What is the 4 digit restrictions passcode when resetting an iPhone?

Then you'll need to restore it through iTunes on a computer to delete everything off it and bypass the restrictions passcode requirement.

Sign out of iCloud first, and turn off Find My iPhone, then follow the steps here: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

It is whatever you set it to. There is no default restriction passcode and it. can't set itself.

That would be the passcode you set up when you set up restrictions. What are you trying to do and what error message do you get?