I have the 8th gen coupe with a 6 speed, and I will drive around town at 40 mph in 6th. Particularly if I know I don't need to accelerate anywhere. I can get about 430 miles out of a tank of gas if I'm not hard on the accelerator, and I believe it's partly because I use 6th so much.
4th keeps the engine RPMs way too high for 40 mph putzing around town when you don't need any power. Waste of gas and wear on the engine. You should use at least 5th but go ahead and use 6th.
Also, I have no problem downshifting when I do need to accelerate - that's why I bought a stick to begin with. So in an instance I'm cruising around and I need to hit it, I'll downshift to 3rd or 4th if I need to.
But when you're on the freeway you should definitely run it in 6th. Any constant speed over 40, you can use 6th.