They'll stay fourth unless one of the three loses again or they beat a top team.
Which is unlikely, looking at their opponents: Seattle (decimated by injuries), Cincinnati (just plain bad, even when Palmer is playing), and Baltimore. The Ravens are decent and can win, but the odds aren't great.
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I agree with Mik3, to go 4-0 is an accomplishment, but if you look at the teams they have faced...the only one notable in my opinion is the Jaguars. And they play the Cardinals next week, which is MORE notable, but not an extremely standout team either. Plus, the loss of Boldin (I dunno how his name is spelled) will be a setback for the Cards, not a huge one, but a setback nonetheless. So yeah, unless one or more of the teams ranked above them lose, I don't think the Bills will advance in the rankings. If anything, I wouldn't be suprised if the Redskins overtook them, if they win next week against the Eagles.
To be fair, the same can be said of Tennessee as well. They're opponents are 4-12 so far. If the Redskins win against they Eagles, they should because it means they've beaten Dallas, New Orleans, and Phili with two of those wins being on the road. It would also mean they have a winning record in what is viewed as the leagues toughest division.
The Bills should beat the Cardinals though. Buffalo has a good defense and Arizona looks turnover prone on offense coupled with a terrible defense. They look like a poor man's Denver Bronco's at the moment: high flying offense paired with a crappy defense. I honestly would not be surprised if they ended up with a first round buy. They're a good team in a mediocre division that gets to feast on the NFC West. It's too bad they don't play Tennessee. That'd be a good chance or both teams to see what they're really made of.