https://www.gamespot.com/articles/fortnites-week-5-challenges-delayed-now-coming-thu/1100-6459258/

Fortnite players will have to wait a little longer than usual for the next set of weekly challenges to complete. Rather than rolling out the new challenges on Tuesday, as it typically has for the past several weeks, developer Epic announced on Twitter that the rest of Season 4's weekly challenges will instead arrive every Thursday, as they previously used to. [Update: The 4.3 update is now available and Week 5 challenges are set to launch on Thursday as scheduled.]

The new set of challenges are now slated to go live across all platforms--PS4, Xbox One, PC, and iOS--on May 31 at 1 AM PT / 4 AM ET / 8 AM GMT. Epic says the delay isn't due to any technical issues, but rather is an "intended change" to the weekly challenge schedule.