How to Enter Diagnostics on a MacBook Pro M4

Use Apple Diagnostics to test your Mac - Apple Support

but you may actually want this article, and the links to related articles it provides:

If your Mac battery status is “Not Charging” - Apple Support

What does the MagSafe3 LEDs show?

Try using a USB-C charging cable instead of the MagSafe charging cable to see if that makes any difference.

Here are two Apple articles for troubleshooting charging issues....they will overlap somewhat, but there are differences due to the different type of charging methods.

If your MagSafe cable or power adapter isn't working - Apple Support

If your USB-C power adapter isn't charging your Mac laptop - Apple Support

Here is an Apple article for accessing the Apple Diagnostics:

Use Apple Diagnostics to test your Mac - Apple Support