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:

Season 4 Week 6 Blockbuster Challenge Loading Screen in Fortnite Battle Royale

If you look closely, you can see the Battle Star in the background, as seen below:

Season 4, Week 6 Blockbuster Loading Screen Battle Star

Reddit user AndrewZ7 has found the star on top of a Fork & Knife van at Greasy Grove, located at C7 on the map and can be seen on the map below, created by SquatingDog:

Season 4, Week 6 Blockbuster Loading Screen Hidden Battle Star Location map
Simply interact with the Star to receive 10 Battle Stars/full tier:

Season 4, Week 6 Blockbuster Loading Screen Hidden Battle Star Location