Why my iphone doesn't download apps?

This is iphone 3gs, it does not download apps from store, it says ios 5 is needed, i updated it few times but the last update is 4.2.1 not higher than this, it also says jailbreak and cadiya, i don't understand what to do & how to get apps from store, please help

You can use this tool to search for compatible apps in the iTunes store:
https://sites.google.com/site/iossearch/

Since you have iOS 4.2.1, leave the menu on iPhone 3G.

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