Well it's like this...
4,000rpm is great for urban scooting. You're already well into the turbo, fuel is ok, you're up to 50-60km/h in mere seconds. Mr. VN Commodore is still messing with his torque converter.
5,000rpm is good for higher speed runs say merging speeds in hwy. ramps. Do you want that semi trailer to just disappear? No problem.
6,000rpm is getting serious - you're WELL over the speed limit, the car is into the power band, everyone around you knows to just stay away, they all move out of the right lane because something VERY FAST is coming thru. Fuel consumption is out the window.
7,000rpm is WSID drag strip or 'da runs bro'...
T.