Actually, they're all baritones except Paul (who's a tenor, obviously); it's more that Ringo has the smallest vocal range.
They probably COULD have added a fourth vocal on some of the songs Ringo sings, since George most likely sang on all of them as well (whether it was a third harmony part I don't know, though), except "What Goes On"... but it's probably not that big of a deal. I mean, if we only get ONE regular guitar (as opposed to bass) part per song...
So do the pitches of their voices go:
Paul
George
John
Ringo?
If you have to ask, then yes.