He's not trying to beat Apoo. He's trying to conserve his strength, that's why Luffy and Zoro tried to run away last chapter.
Using Gear 4 would obviously take a lot of his strength pointlessly. There would be no point in taking down Apoo if you lose the strength to take down Kaido.
Someone else can take care of Apoo. Someone else can't take care of Kaido.