Rent in advance

I am in the process of renting out a property and someone has come forward who, for a small discount on the monthly rent, is willing to pay me 4 months rent in advance (plus one month deposit, which I will put into the DPS). The initial AST will be for 6 months.

Are there any "gotchas" with this? I usually use a 6-month AST, with rent paid in advance. Can I use an AST still, or is there some other mechanism I should use to set up the tenancy agreement?

Thanks in advance.