Stuck Designing Two-Layer PCBs? Give Four Layers A Try! | Hackaday
Eagle Layer Setups
How to access a 4 layer board in EAGLE and Fusion
Using EAGLE: Board Layout - SparkFun Learn
Homemade Hardware
Selecting your PCB Layer Stackup in Autodesk EAGLE | Set up your Layer Stackup for a multi-layer PCB design in EAGLE. Try it in EAGLE https://autode.sk/2CyktqH | By Autodesk EAGLE | Facebook
Scott's Z80SBC Part-1: 4-layer PCBs with Eagle and JLCPCB - YouTube
Review request - First 4 layer board attempt : r/PrintedCircuitBoard
Four or more layer PCB design for beginners using EAGLE
power electronics - Routing traces of different width onto an IC on a 4-layer PCB in Eagle - Electrical Engineering Stack Exchange