I would like to know what could happen to my iPod if it can't handle the update to iOS 6.1.5. It is currently on version 5.1.1 (9B206).
There's no harm applying the available updates to your device, as only compatible updates are shown.
The latest iOS version supported by 4th generation iPod Touch is 6.1.5 you mentioned. After the update, there shouldn't be any available updates to apply.
I use the original iPad as an example. The newest iOS version for it is 5.1.1 and iTunes gives the following message when
Check for update button is clicked and the iPad already has that version installed:
This version of the iPad software (5.1.1) is the current version.
Trying the Over-the-Air
Software Update on the device itself tells the following:
iOS 5.1.1. Your device is up to date.