If I've got it right :

Over their history, Supremes singles can have an assortment of people vocalizing on them , sometimes a whole lot of extra voices can be heard --- and sometimes there might be as few as zero Supremes performing [[if you decide that from '67 onward Diana Ross was not herself a Supreme).
The earliest recordings featuring four Supremes did not result in any hits. Thereafter, for years, the only certified Supremes voices heard were limited to a maximum of three. But as replacement members checked in and out and in again , the total number of certified Supremes accrued. And there was at times some overlap.

In this process , an example of a Supremes HIT [[top 40) that has four certified Supremes singing on it is : I'M GONNA LET MY HEART DO THE WALKING
The record has the voices of the 3 current Supremes: Mary Wilson, Sherrie Payne and Susaye Greene---- plus the residual vocals from Cindy Birdsong....

from wiki :All of the tracks had been completed with Birdsong, but Greene was brought in to overdub her vocals on two tracks. Among those two were "I'm Gonna Let My Heart Do the Walking".

So when you add them up , that's four Supremes singing on there.....


I'm guessing this is not the only such occurrence ??
And I'm sure someone here will be able to fill in the blanks.