Why isn't there iOS 7 update on iPod 4th generation??

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iOS 7 requires 512 MB or more of memory, and the 4th generation iPod touch has got 256 MB

Yes maybe

There is nothing you can do! Not unless there's a technical flaw will there be an incremental update but never iOS 7. iPod Touch (4th gen.) has reached the end and update 6.1.3 is the end. iPod Touch (4th gen.) has only 256 MB of ram and isn't enough to run iOS 7 whereas iPod Touch (5th gen.) has 512 MB of ram. Therefore if you want iOS 7 you must buy iPod Touch (5th gen.)