Will unplugging MacBook Air M4's charger when the charging LED turns green affect the computer?

Butterfly_Kite_And_Bee_Kite wrote:

What happens if I unplug the charger of my MacBook Air M4 when the charging led on the charger turns green and leaving it unplugged until the morning, what will happen to the computer?

Nothing will happen to the computer. Though, if you don't shut down the computer, then it will slowly deplete the battery.

Also what will happen if I use an extension cord from a MacBook Air (13-inch, 2017) to put onto the charger of my MacBook Air M4 chip?

If it actually fits, nothing will happen.

Will it damage the computer or not?

No.

Here is Apple's guidance on the subject of battery care:

Batteries - Maximizing Performance - Apple Support

About battery health management in Mac laptops - Apple Support