It would be fun if we could do this from memory, without having to resort to searching the web for it, but let’s see how many of those old quartets we can name (and even post links to) from the past. FOUR has to be part of their name, though.
Groups like:
Four Freshmen
Kirby Stone Four
Four Lads
Brothers Four
Folk, Pop, Jazz, whatever…
The Four Neat Guys. 
Whoops, sorry! That should be The Five Neat Guys!
Mea culpa!The Four Seasons.
The Four Tops
The Bobby Fuller Four
The Four Lovers 
The Fab Four! Okay, that doesn’t count.
Damn close though! 
I suppose jazz quartets don’t count either. Or four-member groups like The Ames Brothers or The Ink Spots. :mad:
The Four Freshmen
4 Non-Blondes (ok, they aren’t from way back when :o)
Not to be confused with Robert Fuller.
I only mention this because I’ve had people swear they’re the same guy. :rolleyes:
Yeah, almost as soon as I posted the thread I realized so many of the great groups were brothers and such. I’m not too picky, so toss in the ones you really liked. For instance,
The Hi-Los
The Crew Cuts
The Del-Vikings (not a “four” in sight!)
Manhattan Transfer
And a group that has way more than four but is extremely hard to beat:
Two more, both relatively obscure Liverpool bands managed by Brian Epstein:
We may be getting near the end of the Fours. How about threes, fives, etc.?
The Tennessee Three/Tennessee Two
The Three Suns
The Three Degrees
The Dave Clark Five
The Five Satins
In their pre-film days, the Marx Brothers performed vaudeville (including singing) as The Four Nightingales.
By all means! The topic was too narrow as defined in the OP, so just go with what feels right.
Gang of Four
Who is not to be confused with Robert Fuller.
three dog night
third eye blind
three doors down