MacBook Air M4 charger heating issue

Physics. The adapter is really a transformer – it takes alternating current (AC) and transforms it to the direct current (DC) that your Mac needs. Like any energy conversion process, it's not perfectly efficient and the energy lost is emitted as heat. The more power being transformed, the more heat. A 140 W adapter that is fast charging a recent MacBook Pro will get warmer than a 30 W adapter that is charging a nearly-full MacBook Air.

Note that 'warm' is normal. 'Hot' (as in, you cannot leave your fingers on the adapter while in use) is not normal and could indicate a problem with the adapter or the wiring/outlet into which it is plugged.

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Very little to go on, I'm guessing your issue is that your AC adapter gets warm when the Mac is being charged. If that's what you mean, that is normal.