How to make iphone battery last 8 hours?

I have an iphone 4 (yes it's pretty old) and I'm going on a sleep away trip for 4 days and it's a 4 hour bus ride and there are no outlets there so I need my phone to work on the bus ride there and the bus ride back any ways to make my phone battery last longer? It usually last 3-4 hours with constant use.

Unless you have Harry Potter as a friend you don't, unless you turn your phone off for every hour which it is one.