A 4X scope works fine out to 300, provided your target is a big game animal, big game is usually easy to see, at 300, even without magnification. I have a 2X scope on my .375, and I can squeeze decent groups out of it at a quarter mile, never mind 300. At 300 I can see and hit a pop can on a still day, and I'm half blind. The advantage of any scope is not it’s magnification; the advantage is that your target and aiming point are in simultaneous focus. There's not much wrong with the 3-9X, but there is absolutely nothing wrong with a fixed 4X either. In fact the fixed 4X will be smaller, stronger, and maybe even clearer.
I think sighting in 2" high at 100 is good advice. A fast pointy bullet like a 150 gr Ballistic Tip at 2900-3000 fps will reach out there, requiring little hold over at 300.