Unlike a ghost-ring rear sight, which has a thin border which becomes pretty much invisible when you look through it, the CX4 has a thick wide aperture disk with a tiny hole in the center, plus you have the protective "wings" on either side which obscure the target even more.
Now, please tell me that you are not trying to use this sight set-up with one eye only! Shooting with both eyes open pretty much solves the problem, and allows the sights to be used very quickly...much faster target acquisition than any open sights. Opening up the diameter of the aperture, as stated above, will improve speed even more. It will also decrease acccuracy slightly, at least in theory, but at the ranges this gun will be used you will never see the difference. Remember, you are not looking at the rear sight, you are merely looking through it. Don't attempt to make any effort to centre the front sight in the rear aperture...just put your eye to the sight (as close as comfort allows), put the front sight on target, and shoot. Once you get accustomed to aperture sights, you will love them.