Q: cannot remember my 4 digit pin number to open my ipod

Forgotten Screen-Lock Passcode
Connect the iOS device to your computer and try to make a backup
iOS: How to back up
https://support.apple.com/en-us/HT203977
Then restore via iTunes. The iOS device will be erased. Place the iOS device in Recovery Mode
https://support.apple.com/en-us/HT201263

if necessary to allow the restore.
If recovery mode does not work try DFU mode.
How to put iPod touch / iPhone into DFU mode « Karthik's scribblings
                     
https://geekindisguise.wordpress.com/2009/07/16/how-to-put-ipod-touch-iphone-int o-dfu-mode/
For how to restore:
iTunes: Restoring iOS software
https://support.apple.com/en-us/HT201252

To restore from backup see:
Restore your device from an iCloud or iTunes backup - Apple Support
https://support.apple.com/en-us/HT204184
If you restore from iCloud backup the apps will be automatically downloaded. If you restore from iTunes backup the apps and music have to be in the iTunes library since synced media like apps and music are not included in the backup of the iOS device that iTunes makes.
You can redownload most iTunes purchases by:
Downloading past purchases from the App Store, iBookstore, and iTunes Store   
https://support.apple.com/en-us/HT201272
 
If problem what happens or does not happen and when in the instructions? When you successfully get the iPod in recovery mode and connect to computer iTunes should say it found an iPod in recovery mode.

Be careful- if you put the wrong passcode in more than six times, it will sometimes lock itself for 21+ million minutes- around 40 years. I know because it happened to me and I am still trying to find a way into it. It had a lot of valuable data on it and hadn't been synced.