There's three different approaches you can take, which are analogous to translating literature from one language to another.
(1) Word-for-word translation. Attempt a direct 1:1 conversion from 4E game mechanics to Pathfinder. This is the most time-intensive but produces the most faithful results. Wherever there is a close analog between two game effects, use the Pathfinder version if it comes close. For any 4E game effect that has no real Pathfinder equivalent, just bring it in as a completely new ability and try to figure out how it would mesh as-is with the rest of Pathfinder's rules. It will be good to avoid adding a lot of brand-new material because you want to play Pathfinder with maybe a little 4E flavor added, not 4E that has Pathfinder rules added in.
(2) Concept-based translation. Take your 4E game effect, monster, or character and break it down into plain language what flavor it is supposed to have, what it is supposed to capable of, or what concept it represents. Then, build your converted object from scratch using Pathfinder conventions and rules, based on your non-game-specific notes. Your conversion will not be a literal one and there may be important differences that will cause some of the original flavor to be "lost in translation," but your results will be more in line with how Pathfinder rules are supposed to work.
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