Turnover time on the planet of Fortnite is 4:00 a.m. Eastern. Season 5 likely won't begin at the specific moment, however. This is a big upgrade, and will probably need some downtime to execute. So Season 5 will begin whenever downtime ends, which is usually approximately 1-2 hours once it begins. https://www.mmogo.com/Fortnite/Items.html
When that occurs, expect a few things to go Fortnite materials down. The most obvious will be those map changes: we do not know exactly what's going to happen, but now we've got a pretty good idea