It's a pretty big difference...
6 has horns that
curve over his face, Bahamuts do not do this in the slightest. It's the same shape only if you twist Bahamuts horns a full 90 degrees... Meanwhile we can't get a clear idea of 4s horns because the angle just isn't appropriate to judge them properly, but if we looked at Bahamut from the top down? I imagine his horns would look fairly similar...
Heck, it's not even the same horn shape... 6 has a much more pronounced curve in his horn than Bahamut... Crappy editting powers activate;
Bahamuts horns are fairly flat and curve down and back along his head. 6 has horns that curve right over his face then go back fairly far away from his neck... They are extremely different...
Scribbling on this again;
4 matches Bahamuts wings, his hand, and possibly his horns, since 4s horns are going back along his head and then curving, the angle isn't great to tell exactly how, though.
6 doesn't match on horns, since Bahamuts don't curve over his face at all, and the hand seems wrong to me. Can't see his wings at all, so who knows.
Last post from me, but I just went into Coil to grab a shot of Bahamut from a similar angle;
-Similar hand placement.
-Similar snout/eye placement.
-Primal Bahamut got bigger wings and put on some weight. That chest doesn't match
any of the Celestial Dragons, so don't even bother.
-Though the artwork has the horns being more flake-y(?), you're sitting here telling me that the Allag mutilated him and that Tiamat said the Primal version was different, so whatever. 6 still has horns curving over his face while Bahamut has no such thing. Both 4 and Bahamut have horns with a similar placement.
-Back scales look pretty damn similar to me.