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Initially ios 8 on the iPad 4 was laggy and a right chore to use but with 8.1.2 it's nearly there. Just the gestures to fix eg somestimes the five finger close refuses to work.
 
Considering that the iPad 2 shipped with iOS 4 and still has iOS 8, there is an almost certain chance that the iPad 4th gen (shipped with iOS 6) will support up to at least iOS 10!
 
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