Has anyone experienced punching the gas and letting go of gas and noticing rpm remaining high at around 4000 rpm? I can’t help but think there is a issue,

 

This is just a characteristic of the car. I'm not sure if its also other AMG models but the G63 likes to hold a higher RPM after you've already gone up there. Most likely it's holding a higher rpm to let you accelerate quicker if you decide to go back on the throttle. If you don't like it when it holds the RPM's up there, you can just manually upshift with the paddles to 8th or 9th gear and after a few moments it will take back over to Automatic mode