Apparently, there was a falling out...
Pete's always been a fairly jealous cat, always butting in the way when anyone dares pay attention to the other three. Recently, we've had this invasion of stray cats entering the yard, and as a result, Pete's also become very territorial. Now, if you pet the other cats, he'll develop a grudge, and start attacking them.
Now Fuzz is very fat, and probably the most laid back animal I've ever owned. When Pete jumps her, she'll smack him off, and be no worse for wear. Polly is a much smaller and more sensitive cat. She started becoming stressed out just being around Pete, always fearing he'd jump her. She quit eating, and started using the cat box constantly. I decided to let her inside because of this.
It worked. After a couple of days, she started relaxing, acting more like her old self. The only downside is I now have fur everywhere.