How do you enter the 4 digit code if you lost your remote

It depends on the model Apple TV. Identify your Apple TV model - Apple Support

The Apple TV (2nd or 3rd generation) need a functional remote to set up the Apple TV Remote app.

The Apple TV (4th or 5th generation) should recognize the Apple TV Remote Control Center widget or the Apple TV Remote app, even without a remote.

For the Apple TV (2nd or 3rd generation), your options are:

• buy a replacement Apple Remote (aluminum);
• temporarily borrow an Apple Remote (aluminum) to help set up the Apple TV Remote app;
• restore the Apple TV and connect a wireless keyboard to help set up the app;
• restore the Apple TV and connect the CiderTV Mac app to help set up the app.

See if this Apple support page can help.

https://support.apple.com/en-ca/HT208492