Quoting myself from somewhere else:
{quoted}
[...] the reason why the point of Yes is 4 people and not 3 is because the team only has 5 people, which let's be honest, the opinion of 3 people vs 2 isn't that big of a difference, and it is the same reason why we wouldn't put too much credit on the voting integrity of a choice made by 3 people where the outcome is 2 vs 1 (though it may be a slightly different story if the total number of the voters are even, but that's not the case here).
Not to mention, voting integrity aside, Riot also intends that games are supposed to be won by destroying nexuses. The voting is supposed to only pass when the game is unwinnable and there to cut unnecessary time wasting, which to that end Riot believes that if 4 out of 5 people believe the game is unwinnable, then it probably is really unwinnable. Hence the vote's passing benchmark is set at 4.