The new loading screen you unlock for completing the first 2 blockbuster challenges reveals the free battlestar location. The coordinates for the Fortnite Battle Royal map read A4, B4, A3 and B3, which marks the exact location for the new battlestar location.
To pick up your reward for completing all challenges simply travel to very west of the map, just underneath Haunted Hills and find the battle star sitting next to some trees.
Season 4 Challenges
Fortnite | 14 EASY Chest Locations WEEK 8 "Search 7 Treasure Chests in Single Match"
Fortnite | WEEK 8 Battle Star Location (Search Between a Bear, Crater, and a Refrigerator Shipment)
All 7 Secret BATTLE STAR Locations | Fortnite (All Blockbluster Stars + Loading Screen Challenges)
Secret Battle Star Location WEEK 7 | Fortnite (NEW Blockbuster Outfit - Blockbuster Loading Screen)
Fortnite | Finding ALL Soccer Pitch Locations WEEK 7 (Score 5 Goals in Different Pitches)
Fortnite | Best Mushroom Locations WEEK 7 (Finding Mushroom Filled Areas | Week 7 Challenges)
Fortnite | Battle Star Location WEEK 7 "Follow the Treasure Map in Pleasant Park" (Week 7 Challenge)
NEW Secret Battle Star WEEK 6 Update 4.4 | Fortnite (Blockbuster Challenge / NEW TRUCK LOCATION)
Score GOALS in 3 different PITCH locations WEEK 7 | Fortnite (NEW Soccer Stadium / Pitch locations)