My Macbook Pro M4 drain its power during sleep mode even it's PLUGGED IN!

What you’re seeing is almost always Battery Health Management doing its thing, not a hardware fault. On macOS Sequoia, your Mac won’t constantly keep the battery pinned at 100%, it deliberately lets it dip (sometimes to the low-90s or even ~80%) to reduce long-term cell wear. Even while plugged in, it may “exercise” the battery by running it down a bit, then topping it back up later. That explains why you saw 96% before, and why re-plugging sometimes forces it back to 100%.

If the drop is fast and goes well below 80%, that’s not normal, Safe Boot and keeping Sequoia updated to the latest (15.6.1 right now) are good first steps to rule out a software quirk. If it still drains aggressively or won’t charge back properly, then it’s worth getting Apple to run a hardware diagnostic.

<< In general, do you mean that my mac is running normally and I don't need to care about the percentage if it's over 80%?>>

You don't need to worry. The charge level may hover at around (not over) 80 percent, and that is normal.

In addition, the lights on the MagSafe Power Adapter are set by commands from the COMPUTER. The Charger is in the computer, not in the Power Adapter.

If you use your computer almost always on the power adapter, and stop fiddling with it, you will see the typical power level seem to hover at about the 80 percent level. Manually manipulating the charge level by disconnecting and reconnecting the power adapter will ultimately REDUCE the battery lifetime, not extend it.

In general, you should ALWAYS connect a power source when it is possible to do so, and only run on batteries (which could be somewhat slower) when no power sources are at hand. Modern Macs maintains optimum battery charge levels under program control, and will NEVER over-charge. Activate Battery Health Management and do not spend another moment of your time thinking about charging.

Wait, did you see it go substantially BELOW 80 percent?

I do not expect there is any problem with your computer. I think you just caught it giving the battery some exercise.

Connected to Power is NOT necessarily charging.

The CHARGER is inside the MacBook Pro, and is completely under program control. Your Mac accepts some power when needs to run, and accepts more power when it intends to charge its internal battery. A power source like the Power Adapter or certain Displays can not 'force itself' on your MacBook Pro. Only the algorithms inside your Mac can decide when is a good time to charge the battery. Your Mac can NEVER be over-charged.

Catalina software 10.15.5 and later for MacBook Pro with T2 chip (2018 models and later) includes a feature called Battery Health Management. Now, based on your usage patterns, this widens the hysteresis to initiate a charge cycle at a lower level, and stop well before 99 percent. 

About battery health management in Mac notebooks - Apple Support

About battery health management in Mac laptops - Apple Support

Battery Health Management feature tracks your usage history and detects that you do not generally run your battery down to low levels. So when you see less than 100 percent as a full charge, it is charging less than the highest level for longest battery lifetime.

NOTE that charging to 80 percent is a side-effect, NOT the GOAL.

The goal is to have some hurly-burly (percolating activity) around using your battery, either by your actually using it, or by leaving it less than fully charged, so that the cells are not 100 percent charged 100 percent of the time. Battery Health Management will also from time-to-time let the battery decline slightly to 'get some exercise' and run lower (thought to be around twice monthly) if you do not do so by your ordinary use.

The feature tracks you actual usage. If you continue to not require 100 percent charge for a few weeks, it generally will revert to a lower "full" level. but it takes some experience, which takes some time

If your recent usage patterns demand top battery capacity, the battery will charge completely to be sure you are not stuck out in the wild with no power.

A SafeBoot Start up your Mac in safe mode - Apple Support will sort many anomalies

Then Reboot as normal and test.

Verify your software is up to date…

The current stable release of Sequoia including bug fixes, security updates is macOS 15.6.1

Keep your Mac up to date - Apple Support

Keep your Mac up to date - Apple Support

I wouldn’t hesitate to take it to your local Apple Retail Store and have it inspected before an inconvenience becomes an expensive repair.

Click ▶︎ Get Service.

Good luck! 👋🏼😉