I read that men have four voice parts,
[ul]
[li]Countertenor[/li][li]Tenor[/li][li]Baritone &[/li][li]Bass[/li][/ul]
where women have only three,
[ul]
[li]Soprano[/li][li]Mezzo-soprano &[/li][li]Contralto[/li][/ul]
When women sing in a quartet, what is their fourth voice?
Sweet Adelines is an organization promoting women singing barbershop style music. The breakdown of the vocal parts they use is LEAD, TENOR, BARITONE, and BASS.
Which is the same for male barbershop quartets. The vocal parts do not strictly correspond to the voice categories listed in the OP. A quartet may include two tenors, who sing different parts, a baritone, and a bass.
Women’s vocal parts are somewhat arbitrary in any case. There’s a pretty smooth transition from soprano to mezzo to alto, and the division is more about what range the song writer wants to put the song in, than anything about the vocal ranges of the singers. Apart from that it’s easier to find someone whose range encompasses a high E, than a high A.
I understand that it’s a bit different with men - that the countertenor is a distinct ‘voice’ of someone whose voice otherwise might be quite deep. But I haven’t known many counter tenors - they’re medium rare too
All vocal parts are somewhat arbitrary. There is a tradition of dividing women soloists into soprano, mezzo, and contralto, and there is a tradition of dividing mixed-voice choir into four parts (soprano, alto, tenor, bass) and a fuzzier tradition of dividing both all-male and all-female choral arrangements also into four voices each (soprano I, soprano II, alto I, alto II) / (tenor I, tenor II, baritone, bass). Mixed-voice choirs are sometimes split into six or eight parts as well. It’s not because there’s really a strong case for an inherent “break”. You could write for seven voices. Five voices. Whatever. (And some composers do).