Either you’re very unlucky in getting two bad units or you’re far more discerning than the tons of other S4 owners who’ve never noticed such a problem.
So go buy one from a place that allows returns and see how your luck is.
Uniformity inconsistencies never stopped being a ‘thing’. Almost all LCD/OLED displays will have some type of disparity in the display, it depends on the background color image that exhibits the unevenness for others to notice it. Plus, it sounds like you partially trained your mind to look for the uneven displays, so you’ll likely look for it again versus someone who would never know what to look for.