A HMO has a minimum of three tenants, who form more than one household, who share a bathroom & kitchen. You will need a licence from your Local Authority if the building has three or more storeys or 5 tenants who form more than one household, so if you let to 4 tenants and the building is less than 3 storeys you will not need a licence.
You will be responsible for health & safety in the HMO and there are quite a lot of regulations, which may vary from one LA to another, get some information from your LA. I am intrigued by your comments on how the rent is paid, where did this information come from?