Oh Naughty Dog, so cheeky and clever with its references. If it's not giving you a PlayStation Trophy for standing still (an in-joke about a flubbed E3 demonstration), it's sneaking in a piece of collectible treasure in Uncharted 4 that alludes to the studio's previous game: The Last of Us. Behold:
And because I like you so much, I'm going to tell you how to get it.
Don't worry, I'm not going to get into spoilers, unless you consider "there are puzzles and skeletons" to be a spoiler, in which case… uh, sorry. Guess I kinda just spoiled it for you. Still, this is a warning that starting next paragraph, there will be a description of where the treasure is, as well as pictures of said treasure. Okay? Don't say I didn't warn you.
The treasure is called "Strange Pendant," and you can collect it in Chapter 9. After you solve the water puzzle and open the door to the next area, follow the path to the right. Search along the right side to spot a hidden grapple point and use it to swing over to a cave on the far side of the cavern. There, you'll find a skeleton with a sword and hat, and the Strange Pendant on the floor next to it.
What makes the pendant so strange to Drake but familiar to The Last of Us players is the emblem; that's the crest of the Fireflies, a faction important to TLOU's plot. It's also, interestingly enough, the exact same pendant you collect in that game. The name on the back - Druckley - is a combination of TLOU and Uncharted 4 directors Neil Druckmann and Bruce Straley.
There are way, way more treasures scattered about the world of Uncharted 4. If you want more help, check out our Uncharted 4 collectibles guide. Or, if you want to catch a glimpse of what didn't make it into the final game, check out this video that explores what was left on the cutting room floor: