But in order to make this case, you have to convince people that it doesn't make sense that someone could wield the same Keyblade 4 times when it has been established that two people can wield the same Keyblade at once. Sora even turns his own single Keyblade into weapons with multiple components (including dual guns) when he does the transformations. It's less consistent to assert that this singular restriction should be taken seriously and constitutes an important sticking point in the lore when we see no less than three people using presumably the same Keyblade in the same fight in KH3, one of whom also wields a second one he shouldn't have according to that very same logic.
There's no reason this would have had to happen. Xion and Roxas both wield the Kingdom Key, though it takes some time for both of them to synchronize its usage, and Xion, Roxas and Sora all use the Kingdom Key without any issue in KH3. There is no real reason Sora couldn't use Xion's Key, Roxas's Key, Ventus's Key, and his own Key, even after they're separated.
Whether it would have made for superior gameplay, I definitely prefer dual wielding to transformations (and definitely AFs), so I'm inclined to think quad wielding would also be better, but honestly I think a KH3 that just removed the gimmicks altogether would have been preferable. Sora's supposed to be weak again. Give him a Keyblade and well balanced magic, maybe some shotlocks and properly useful summons as the game progresses, and call it a day. Most importantly, give him the best and most reliable allies. I would have much rather they focused on the party system and building something with that which could have come into play in a big way at the endgame.