UPDATE: This hidden Battle Star has been moved in the V4.4 update to the new Soccer Stadium, as seen below, posted by Reddit user OneshotFire:
Season 4 brought a new feature to Fortnite Battle Royale, Blockbuster Challenges. Players are required to complete all challenges for the week to complete the Blockbuster Challenge, unlocking a loading screen, which contains a clue as to where a hidden Battle Star can be found on the map.
Here is the Blockbuster loading screen for Week 6:
If you look closely, you can see the Battle Star in the background, as seen below:
Simply interact with the Star to receive 10 Battle Stars/full tier: