I forgot my four digit restriction code from Apple TV. How do I find out what it was

You will need to reset the device to factory settings, if you can’t guess it.

Reset or restore your Apple TV to its factory settings - Apple Support

or if you can’t access that through the menus: Restore your Apple TV through iTunes - Apple Support

Settings > General > Reset > Erase All Content and Settings

That will ask for the restrictions code however, so will not work.

From

Use parental controls on your iPhone, iPad, and iPod touch - Apple Support

If you lose or forget your Restrictions passcode, you need to erase your device, then set it up as a new device to remove the Restrictions passcode.

If iTunes is not an option, then erasing the iPhone through iCloud may be the only option:

Erase your device with iCloud to remove your passcode

If you set up Find My iPhone on your iOS device or Mac, you can use iCloud to erase and restore it. This will erase the device and its passcode.

1. From another device, go to icloud.com/find.
2. If prompted, sign in with your Apple ID.
3. Click All Devices at the top of your browser window.
4. Select the device that you want to erase.
5. Click Erase [device] to erase your device and its passcode.
6. Now you can either restore from a backup or set up as new.

If your device isn't connected to a Wi-Fi or cellular network, you can't erase it with Find My iPhone.

Best of luck