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I like to believe that Pokemon can use as many moves as they learn, but they have to keep dedicating time and practice towards a move or it'll suffer a form of magical atrophy and fail to be as battle-ready/potent as it should be. 4 being the most the average mon can handle.
Moves that aren't kept up to standard can still be used but are a lot less powerful or have weaker effects. So a Pokemon can still use, say, Rain Dance but it'll only be a tiny little cloud for gardening instead of clouds shrouding the skies.
