A lady in a YouTube video says "fourth varna" to refer to the varna said to be born from Purusha's feet. Does scripture use this euphemism anywhere?
I think you are looking for something like this?:
To a Vaisya pertain in addition agriculture, trade, animal husbandry, and lending money on interest.
The Sudra is the fourth class with a single birth. Speaking the truth, refraining from anger, and purification apply to him also. 52According to some, he should simply wash his hands and feet in place of sipping water. 53He should make ancestral offerings; 54support his dependants; 55be faithful to his wife;
Gautama Smriti 10.51.49-56