Landlords are free to decide the length of the fixed term - we do not impose a minimum fixed term.
It is however important to understand the implications of setting up a tenancy with a fixed term of under 6 months.
Section 21 of the Housing Act stipulates that a landlord cannot legally obtain possession of a property in the first 6 months. In the unfortunate event a tenant refused to leave at the end of a 4 month fixed term for instance, this would pose problems by making landlords unable to legally regain possession. A section 8 notice may however be used within the first 6 months - you can find out more here.
It is worth remembering that in Scotland, there is no section 21 equivalent and one of the grounds of the Private Housing (Tenancies) (Scotland) Act must be relied on. In Wales, no fault notices can be used to end tenancies - however only after 12 months.
Thankfully, tenancies generally end by mutual consent though given how this is enshrined in law, it's always worth being aware of the implications of a fixed term under 6 months.
You are of course able to agree a shorter fixed term with the tenant(s) if that's preferred, and Rent Now will still accommodate this.