I've actually had to do this for my niece's 4XL, so I've experienced your pain. That's why I always buy extras when I get screen or back cover adhesives, because as you found out, it's all too easy to make a mistake and have to start over. I found a YouTube video from Ricks Tech Repairs that shows you exactly how to put the adhesive on. The part you're interested in starts at 10:45 into the video.
Google Pixel 4 XL Back Cover replacement - YouTube
I took a look at the adhesive sold by iFixit vs. the one I used (I still have a couple), and mine is almost identical to the one in the video, but that's not important. Yours already has the center of the adhesive removed, so you don't have to worry about that part. Here are a few pointers to help you along.
- Pay attention to the shape of the adhesive; the top and bottom are different, so you need to make sure you apply it in the proper orientation. The top is a slim and straight across while the bottom is wider and has a small tab in the middle.
- Plug in the battery and make sure all the cables and connectors are laying down flat so when you place the adhesive, the backing doesn't get pushed up and make it difficult to lay the edges down all the way around.
- Yes, the adhesive does go on the main frame, not the back case.
- The big blue sheet stays on, and you remove all the backing from the side that goes toward the phone.
- Lay it in place carefully; if you need to hold it down, press on the blue backing, not the edges - you don't want them to stick until it's properly positioned. On yours, the big opening corresponds to the camera position in the upper left corner.
- Once you're happy with where it's placed. lightly stick the edges down. Then go over them all with a spudger as shown in the video to secure them.
- Carefully separate the backing where the opening is, then work your way around slowly. If it comes up from the case anywhere, use your spudger to press it back down and stick it good before proceeding.
Hopefully that'll get you going. Best of luck with your repair!