One of my tennants in a buddy up house arrangment is having a baby

Tenancy Types and Management

I think you will find if your four tenants are friends and not related then you may already be in an HMO situation.
I suggest you look carefully at the HMO advice/literature available and then work from there. Sort out one problem at a time.
I would also arrange for all bills (gas/electric/council tax) to be in a tenants name - they could leave you in a real mess financially if they chose not to pay you at any time - at least if the bills are in their names then they are totally liable, and its not your problem.
Good luck!