If you are already a Sirius subscriber and you purchased an all access subscription, then you can stream Sirius radio using the Sirius app from the iOS App Store, provided that your 4th generation iPod Touch can use the Sirius app.
Also, the you need to check what iDevice models and what iOS version is required to use the Sirius Radio app for the iOS from the iOS App Store.
The current Sirius Radio app needs an iDevice that has, at least, iOS 6.1 or greater to use the app.
IF you do not currently pay for an all access subscription to Sirius radio, you would need to change your current subscription and subscription fee/payment to all access by accessing your account info on the Sirius XM website.
Good Luck!