That's just it though.
If the Bills were in the NFC, they probably wouldn't of made it to one Super Bowl, much less four. That's how dominant the conference was back then.
I'll submit that the 90's Packers, the 90's 49ers and perhaps some of the 90's Eagle's teams were better than the Bills were. We already know how much better we were then them and I'm not even counting the two other teams they lost to in the Super Bowl in the 90's.
Greatness is a term that gets thrown around alot. I don't think they were great. They played in an far inferior conference. All of the teams mentioned above, except the Eagles (lol), won Championships in the 90's and they had to do it by going through us just like we had to beat them when we won our three.