Every week of Fortnite: Battle Royale brings a new set of challenges for players to complete and earn Battle Stars. When they complete every challenge, there is usually a hidden item somewhere on the Fortnite island that will grant players a bonus that is not listed in the challenges.
This fourth week of season six challenges hides a secret banner players can grab to use on their in-game profile. To be able to see this hidden banner, you must have completed all week four challenges, including those from the premium Battle Pass. Players with the free Battle Pass will not be able to see or grab the item.
The loading screen for week four gave us a hint at where the secret banner would be.
The loading screen shows a llama with a banner on its back. Since loot llamas are not guaranteed spawns in any sense, the only llama left on the map is the editable one north of Junk Junction. And it is quite easy to land on its back.
If the Battle Bus is anywhere near the northwest part of the map, you should be able to land there quite easily. There might be some loot there as well. Just interact with the banner and it should become available in your inventory at the lobby when you finish that match.