Battery % dropping in a very strange way. Please help.

Sorry, but I'm having a hard time understanding what the "issue" is.

You charged it to 100%. over 3 hours later it was at 96%. Seems reasonable... If you're concerned because it was plugged into power at the time, it will do that from time to time thanks to the optimized battery charging logic.

Don't worry about it. it's fine.

Agree with @KiltedTim; this sounds like normal behavior.

Here is Apple's guidance on the subject of battery health & management:

Batteries - Maximizing Performance - Apple Support

About battery health management in Mac laptops - Apple Support 

If battery charging is paused or on hold on your Mac - Apple Support

In a nutshell, to get the most from your battery:

• Let the OS and charging circuits manage the charging.

• Plug in when using the computer near available power.

• Use on battery when ac is unavailable or when you must be mobile.

• Don’t subject the computer and battery to extreme temperatures.

• Don’t allow the battery to regularly discharge deeply before plugging in. Yes, it may happen from time to time, just don't make it a habit.

• Don't obsess over battery life and/or health.  

❝Worry is the misuse of imagination.❞ – Zadra