how do i recover the old 4 digit passcode?

Are you saying that you forgot your passcode? If so try this:

If you forgot your iPhone passcode - Apple Support

If your question is how to go from a 6 digit code to a 4 digit code, I would not recommend it, too easy to guess. However when setting a new passcode you will be given that option.

If you forgot your passcode and are unable to unlock your iPhone, you can erase it to remove the passcode lock and restore it from a backup.

For a complete step-by-step guide, click here.

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