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My wife has the 4 and she did the upgrade a few days ago. No problems so far. She says everything is as good as it was before with a few more features. She's a moderate level user, more than just consumption but not a power user either.

So, from our experience the 4th gen ipad runs perfectly good with iOS 8.
 
I upgraded.

In my opinion, Apple seems to have taken a page from Microsoft's playbook. They change things just for the sake of change and not because it makes for a better user experience.
 
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