I don't see how an AP would not be '4e friendly'. Is Mutants and Masterminds 4e friendly? What about Call of Cthulhu?
With enough effort, any AP can be converted to another set of rules.
In my experience so far, the writing style of the Pathfinder AP's and modules are relatively "generic" enough for moderately easy conversion to other rules sets.
I had one friend who has adapted the Second Darkness AP to the CoC rules for his own game. (This guy was a big fan of both Call of Cthulhu and the Pathfinder AP's).
I don't know if the Paizo people have consciously and/or purposely written their adventures in a relatively "generic" manner. But nevertheless, the writing style and structure of their AP's and modules are "friendly" enough for easy adaptations to other rulesets.
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