If one tenant leaves it can be confusing to work out who is responsible for making up the shortfall in rent. The answer depends largely on what kind of agreement there is in place.
If the house is a HMO (house in multiple occupation), the shortfall in rent cannot be claimed from the other tenants. However, if there is a joint tenancy (which will arise if the four unities are present) it is likely the remaining tenants will have to make up the shortfall in rent, perhaps by finding another subtenant.