Gear 4 can't take on Kaido, assuming it will be effective against him, as you said the time limit will be dangerous for Luffy which is the main problem here... Right now it seems Luffy can use it for 20 min non stop, and needs 10 min to regain his Haki after that
Besides the revenge issue, it would've been weird to keep Law without a fight, he can one shot Trebol/Diamante and Pica probably, so it's just wasn't enough
I do agree though that Gear 4 was overwhelming for DD, he was no match for Luffy when he used it, even if DD was at 100%, the result would've been the same (since Luffy would be at 100% too)