Got a 2008 GMC. Acadia. Battery down after 4 days of not being started. Has a new battery. Charging system charges great.

The first thing to do is have the battery tested and the battery charging system (the Alternator)

Visit the on-line store, know as reliable-store, they have the repair manuals. I'm sure you will get the solution of your problem.
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Make sure battery has a full charge and battery connections all good. You can't do any testing if battery is down. Forget the jumping. Also, check your charging system.
Does starter motor crank? If not, check wiring circuits for the starter motor. Any testing at the starter, make sure tranny is in park or neutral and functional parking brake is set, Safety is your responsibility.
If you have a digital multimeter, you can do some testing. If the battery is down, put a charger on it.
Sometimes if battery voltage is lost, it can effect your security system. I haven't looked to see if it will effect your starter motor?

The battery is either dead, or the battery voltage is too low to power up the lock solenoid in the steering column to release your key. Jump start it, and then go get your battery/charging system diagnosed!

Floorboard is wet because the drain from the A/C Evaporator is plugged up with "gunk" Need to find the drain and clean it out.
For a 2008 the Battery has used up it's 60 month life and you need a new one

Are you talking about low battery voltage, or low charging voltage?

If the charging voltage is around 14v, then the alternator is fine. If around 12.5, then replace.
If the battery voltage at rest is less than 12.5, then replace the battery.

your charging system isn't working, if you replaced the battery and it worked for a few days, then its not being re charged after starting, I'm assuming you cleaned and tightened all the battery post connections when you installed the new battery , if not do so ,, charge the battery from an outside source, once you know its fully charged,, use a meter, and start the car ,, check the voltages at the battery posts,, you should be getting a reading of 13.5-14.5 volts, if not your alternator isn't charging the battery

A car battery life is about 3 years.. If the battery is newer - there could be a problem with the charging sytem.. Have the battery and the charging system tested.

have a great day

check water in battery or replace it or a wire could be touching or if the battery is not stable it will slide and bump metal and die

There is no way Onstar would drain your battery in two days. Electric seats? They only draw power when they are running, same as any other device in your car, except the clock. You have either a ground -fault issue or a dome light is always on. Good luck.