I'm not altogether sure what you mean by 'a 4th' since you've not actually stated what chord you're hearing. But assuming we're talking about the same place, it definitely sounds to me like there's a semitone shift on the last two beats just before it goes back into the verse.
On my second listen it actually sounds more like the bass shifts down from an F to an E. But you could play an Eb and it wouldn't hurt the song much. I can't hear an A at all and such a simple harmony would dull the colour in the rest of the harmonies somewhat. How he manages to write such rich harmonies and then gets lost in the groove I don't know. The guy's a incredible musician.
When I played this track in my own band, I had more trouble with the passage at the end of the first verse. Neither the Musical Director nor I could pin that one down and I ended up having to wing it. (Although the girls ended up hating the song and we dropped it before performing it live). Now I've got a decent set of studio monitors to listen to the tracks through, its a lot easier to pick the changes and I know now that the bass was actually a lot simpler harmonically than the MD wanted me to play! (No wonder I kept stuffing it up eh? Bloody keyboardists. )