It won't hurt to punch it to full throttle and let the car shift for itself in either mode with the gear selector set to 6. If you are in a lower gear and hit redline the engine will cut off until you back off the gas and let the rpm drop. It won't allow you to over rev.
The automatic shift points depend on how far you are pushing on the gas pedal. Normal driving it will shift at the speeds you see. If you mash down on it, it will shift later at higher rpm. There's no reason to drive around in 4 as the automatic will shift to the gear it needs to be in depending on speed.
The paddle shifters are there for you to downshift and get some engine braking, or to control upshifting if you are on a hilly/curvy road and want to stay in a certain gear. Just play around with that and you'll get a feel for it.
I go up and down a 3 mile mountain road quiet often. Going down, if I am going too fast, or if I have slower traffic ahead, I'll downshift to 5 and then 4 if needed to keep the car from speeding up and not having to over use the brakes. That's one example.