It's definitely not four, it's the expletive. I can hear it. Also, Ken Caillat confirmed it in his Q & A on this website a few years ago.

So what was he "f!_!ckin" about before he recorded one of the most legendary songs in rock ever. If it were me, you'd hear a faint "Yippee!" in the background. There's nothing to be mad about when you're miliseconds away from recording "The Chain."

The Case for F#ck. :P
 

Well remember that they all stayed back one night to join forces and make the song work. By the time they finally cut whatever track Lindsey's swearing on, he was probably dead tired, frustrated, high - one can only imagine!!

Besides, the supposed expletive was taken off the later cd pressings of the album. It was on the first pressing. And of course you can still hear it on vinyl. If it exists at all. ;)

Let's add Lindsey's blabbering at the end of "Caroline" to this list. To me it sounds like: "whathaveidone, whathaveidone, whathaveidone". You've done a great piece of work there Lindsey. ;)