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A simple thing you can do would be to just allow 4 pokemon in your party at a time. Another option would be to make a swich that changed the pokemon that are desplayed in your party. For example, you could carry 8 around with you at a time, 4 would be desplayed in the menu system, then if you pushed '1' on the map or something, it switches it so that the other 4 are desplayed.