Usually, Animal Crossing: New Horizons will only allow you to climb up three levels before it says it's too dangerous to go any higher. However, with this glitch, you're able to reach that elusive fourth tier and begin decorating the highest reaches of your island.
The glitch has been around since the 1.3 version update, and as of version 1.9 it's still doable! Read on to find out how it's done!
All credit to Animal Crossing World for the images used in this guide.
Animal Crossing New Horizons: How To Climb To The 4th Level Glitch
Step 1: Dig 3 tiles in and 1 round off 1 tile
Your first step is to dig two tiles across, and then one full additional square deep on one side. For the fourth tile, just round it off so you've got a 2x2 square with one rounded corner, as seen in the image below.
Step 2: Place a small item in the middle of the 2x2 square
Next up you want to take a small, 1x1 item and place it in the middle of the four tiles. It wants to line up so that half of its 'box' is on one square, with the other half on the adjacent square.
Make sure that you're between the item and the cliff you want to end up on top of. Next comes the tricky bit...
Step 3: Face the item, then turn and build
Once the item's in place, make sure you're facing it. Then, immediately turn towards the empty corner square (bottom right / south east in this image) and hit A to build. If you time it right, your villager should fill in the bottom square with some more cliff, filling in the curved corner and popping you up onto the 4th tier.
Step 4: Decorate!
Once you're up there, you can run around and decorate as normal! You can plant trees, bushes and flowers, and even place furniture items.
Your only restriction is the Super Mario Warp Pipes, so if you were hoping to place one so you can get back up without using the glitch, we're afraid you're out of luck.
How To Remove Items From 4th Tier
If you do want to remove any of the items you place up top but can't access it using the glitch, you just need to request an island clean up.
Stand next to the cliff in question, bring up your Nook Phone and select 'Request Cleanup' from the Island Designer app. This will get rid of any trees or furniture items you've left out of reach. Bear in mind that flowers are seemingly unaffected by this however.