So it looks like there was something hidden in the Week 4 loading screen after all. Earlier in the week, it looked like Fortnite hadn't put anything in a relatively oblique loading screen featuring Sandstorm and Scimitar, two upcoming skins for the item shop. Turns out there was something hiding in there after all, however. We got two Fortbytes dropping n the game this morning: Fortbyte #80, which you can read about here, and Fortbyte #91: "found at a location hidden within loading screen #4". Read on for a map, guide and location for where to find Fortbyte #91.
First off, let's look at the loading screen. As usual, you won't be able to see this unless you complete four full weeks of challenges or, you know, google it

The twist, and the reason why we assumed there wasn't anything of note in that image, is because there actually isn't anything hidden in the image. The clue actually lies in the in-game description that you get when you look at it in your locker:

"Sandstorm and Scimtar race to the Keyboard King ATM, and the winner always gets paid".
So there's your hint, and it's a whole lot more straightforward than the image suggests. We're going to Keyboard King, a store in Paradise Palms.

outside of the store is an atm, interact with it

And there's Fortbyte #91. And a note: when hidden battle stars appear in the loading screen, you actually need to complete the challenges to access them. Not so in this case: you'll be able to get it no matter what, so don't worry about slamming out those challenges in order to keep building your Fortbyte collection.