Reducing a 6 inch ac exhaust to 4 inch
Gurji, that’s what what I got also. I did calculate it after making the statement. In basic duct sizing 4 to 6 is double and 6 to 8 is double, but pi r squared is right. But I only do HVAC part time.
Cutting the area in half may speed the air up in the choke point but fans work by moving air--once the resistance increases the blades start “slipping”. I don’t remember the tech term but it’s like cavitation on a boat prop; you can put all kinds of power in but the boat goes slower until the blades bite and push the water air does similar. You can put a tachometer on the fan and watch it speed up but it is moving less air. This why duct sizing/flow is critical to set up a system.
Cutting 10% may not make a huge difference, but at 1/2 you may find your system is no longer going to cool the garage and it may shorten the service life with your compressor possibly shutting down from overheating (this can damage the refrigerant in some units).  Can you go up and out? I would not suggest cutting the flow by that much.