The fourth secret battle star location for season 5 in Fortnite unlocks after you complete any 4 weekly challenge sheets. On the loading screen you get for completing the challenges you can see the pool in lazy links with the secret battle star sprayed on the wall in the background. Next to it you can see the soda camel that can be found in the desert.
Secret battle star number 4 can be found on the back of the camel in the desert.
All Week 4 Season 5 Challenges
Build Structures (0/250)
Jump through Flaming Hoops with a Shopping Cart or ATK (0/5)
Eliminate opponents in Dusty Divot (0/3)
Deal damage with Sniper Rifles to opponents (0/500)
Search Chests in Flush Factory (0/7)
Search between a Gas Station, Soccer Pitch, and Stunt Mountain (0/1)
Pistol Eliminations (0/3)