I think you're getting mixed up between a part 4 tenancy and a lease. If you have signed a lease, this is a binding contract which you are not entitled to break unilateraly - ie you can't kick the tenant out to sell the house. howefver, at the end of the term of the lease be that 1, 3 or 5 years, you can sell the house if you want.
The tenant is not entitled automatically to a renewal of the lease at the end of the year. Whether you both want to sign a new lease at that stage is up to you both.
What you are referring to is the new rule which state that if a tenant has been living in rented accomodation for greater than six months he or she is entitled to remain in there for 4 years once the rent is paid. There are certain rules as to when the tenant can be evicted which include if the landlord wants to sell the house. The purpose of this is to give tenants some security of tenure while protecting the landlord's right to use a property as he sees fit. It prevents the landlord from kicking someone out just because they want to jack up the rent an unreasonable amount