Completion of all Week 4 challenges unlocks a new Blockbuster loading screen with another Battle Star to collect, as seen on the ledge:

The new Battle Star is hidden in ‘Omega’s Lair’ slightly east of Snobby Shores. To collect it, simply visit this location and interact with the star. You will need to build up near the rocket to be able to reach it.

An image of the location in-game via Reddit user VintageCacti:

Note: You must have completed all Week 4 challenges for the Battle Star to appear.