I think a lot of people are thinking along the same lines as you.
If you have 4 properties, why get yourself classed as a large landlord with all the restrictions if you can simply sell one.
Keep in mind though, that as a small landlord if your tenants stay in place, you can only sell once every 6 years. If you have tenants replacing each other over the 6 years (a house share) I've no idea how you'd work out which tenant the 6 years is based on. Also rent increases if the tenants don't leave is HICP/2% and a large rent increase at the end of the cycle probably won't be allowed politically. Being a small landlord has downsides as well - just not as bad as being a large one!