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Go to the app store and go to the top free apps section.

Aside from that, you can get AppOfTheDay, which will give a link to one free app per day. Something like AppShopper will also show free apps, and there is likely a bunch more.

Keep in mind that many games are starting to be unavailable for the iPod touch 4 (it is 4 and a half years old and 2 iOS versions behind), but some still exist.
 
At the time the Touch 4 was rubbish. Honestly, sell on eBay pick up a 5th Gen and starting downloading those awesome games!
 
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