Read How to test a MacBook battery: see if it needs replacing & replace it. Our list starts with ... potential of its M-series MacBook Pro and Air batteries. All the MacBook Air models, whether ...

then microsolder the replacement PCB miniboard into those sockets. This is referenced, but not shown in detail. The video also goes over how the MacBook's battery must be disconnected from the ...

Apple has improved battery life on all M4 MacBook Pro models, and that tells me the M4 MacBook Air should see similar gains.