I agree about Oda's number issues but there have been exceptions as well. On the topic of gear 4th and 5th; to me, all gear 4th forms are on the same level, they are of course tuned to specific type of opponents, but usually that's it. Gear 4th barely works against YCs, even with advanced CoC haki I don't think gear 4th is the solution against Kaido. But then, there's the question of Luffy's issues with gear 4th, ideally Luffy should solve the problems with gear 4th before transitioning to gear 5th, but on the other hand Luffy was using gear 3 without solving the issue with gear 2. I think Oda wants to introduce Luffy's last gear form in this arc, the atmosphere and time is perfect + Kaido is the perfect opponent for gear 5th introduction.