Sorry, I missed the second part of the question.
One of the advantages of a Part 4 tenancy, for the tenant, is that he can give notice to Terminate the tenancy for no reason at all - if he just wants to leave, give the landlord a valid Notice of termination and before the notice period expires, vacate the property. The tenant is entitled to the return of his deposit except for damage beyond normal wear and tear. Rent will be due up to and until the notice period ends whether the tenant vacates the property before the end of the notice period.
With a fixed term lease the tenant may only leave and retain the deposit, by assigning the remainder of the lease to a new tenant and all the costs of finding the tenant are the responsibility of the out-going tenant. Thus, a Part 4 tenancy is easier to get out of, but the Part 4 does have its downside in that the landlord can remove a tenant under 6 grounds or reasons as set out in the RTA 2004.