How to end current tenancy? Can tenants leave without notice?

I would recommend you get something in writing as you should have evidence that the tenancy has ended in case its questioned later.

Even if an HMO licence is not required, you will still need to comply with all the HMO Management Regs and with 4 people get a fire risk assessment. That will probably require an interlinked alarm system and fire doors as a minimum.

Thanks, I will ask the tenants to email me that they are moving out on such and such date.

The 4 people are husband, wife, infant and a friend of them. So, 3 of them belong to one family and the 4th one is not related to them.

I already have interlinked smoke alarms on each floor of the property and also a heat detector in the kitchen. All are wired.
I don’t have a fire door.

You need to get them to serve you a valid Notice to Quit as specified in your tenancy agreement. Just emailing you a date may not meet this requirement.

Thanks @David122, I will ask them to give me in writing.