how do i recover the old 4 digit passcode?

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.

Need more help?

• Chat now or call 1-800-APLCARE.
• International Support.

Click ▶︎ iPhone User Guide. 📖

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.