252, 4 and 252 are not the Pokémon stats, they are suggestions as to how you should train your Pokémon. Since yours is already lv 80, it's most likely already received all of its EVs, under normal training. But if you want to battle competitively, you'd need to start from scratch and start EV training.
Well, if you want to EV train your Decidueye, it isn't too late. You can remove all of his EVs with berries and train him again.