Short term AST less than 6- months Notice

Possession - Section 8 and Section 21

A current tenant wishes to leave the household, and I've given the green light since she found a replacement. Now, I'm setting up a 4-month AST (Joint and Several) until April 24 because another tenant doesn't want to renew after that date.

I've heard that if you create a tenancy for less than 6 months, you can't give notice until after a six-month period has passed. My question is, if the tenant gives notice, let's say in February 24, during this 4-month tenancy to leave in April 24 , are the other housemates also required to leave on the April 24, or do they have the right to stay on ove the preiod of the 4 moth AST, given the short duration of the tenancy?