Making the hole two small will put back pressure on the system side affects are not cooling as well, possible overheating and a shorter service life for the unit. For a given pressure drop, the flow of a liquid or gas through a cylindrical pipe varies as the 4th power of the radius. 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.

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”. But the point, is right, going from 6" to 4" is likely to cause significant performance issues with the A/C.Unless otherwise specified by the purchaser, these tolerances apply to the nominal inside diameter, which equals the difference between the nominal outside diameter and twice the nominal wall thickness. The answers here quoting the difference in cross sectional area underestimate the size of the effect. But it will go thru a smooth metal pipe as opposed to the extendable plastic hose that has the ridges in it and I’m sure disrupts the smooth flow of air. 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). This tolerance may not apply in localized areas of formed fittings where increased wall thickness is required to meet design requirements of ASME B16.

So a 6in pipe will have a flow rate 6

The ac will be about 10 inches from the wall with a 6inch pipe, then go to a reducer to make it a 4 inch pipe for another 12 inches thru the wall and out of the house.