Crappy MSPaint edit ahead;
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I flipped 6 to give a better angle. First thing I notice? His horns go froward over his face then curve back, both the minion (which is still a minion of the Primal) and the Primals only go back, while still curving further back slightly. If you strip away all the gold Allagan augmentations off the Primal, the horns on the minion seem like an exaggeration of the Primals horns, I mean the minion is of Primal Bahamut, and that neck is clearly wrong.
Next lets look at 4s wings. They take up a fair portion of the artwork, and I'd say they're pretty damn similar to Bahamuts, right down to an almost claw like appendage at the tip. Yes 4s wings look a bit... feathery... You know what though? Bahamuts wings are covered in gold, and I'm incline to think all the gold on Bahamut is an Allag augmentation. Can't see 6s wings, but again, 4s are clearly on display here... I think in all cases, the first we see of Bahamut are his wings? When he pops out of Dalamud, it's his wings we see first, when we meet him again in Coil? Right off the bat, t1 shows us his wings.
Next up, the claw. Both 4 and 6 appear to have 3 claws, which is matching with what Bahamut has, 4s is kinda hard to make out, but I'd say that is much closer in alignment to what we fight on in t5 than what 6 has, where all the claws are next to each other almost like a paw. Make a fist with your left hand, then only extend your thumb, middle finger, and little finger; That's what Bahamut has. Make a fist with your right hand, then only extend your index finger, middle finger and ring finger; That's what 6 appears to have.
Also potentially worth noting is the spines along his back, 4s match up with the Primal Bahamuts more closely IMO.
IMO, 4 is definitely closest to Bahamut. We can't even see that much of 6, just his head and a foot... Of course the Primal isn't a perfect representation of Bahamut, and the minion isn't a perfect representation of the Primal Bahamut, but the hand and the wings sell it for me, plus 6 has a beak and his horns are arguably wrong.