I'm a tool freak, so my advice is.... Buy them. Then get another 32" US 4, so you have the complete set of sizes. I like
tools and gadgets.
a few circumstances when you can visibly see the .25mm difference. (Vast fields of st st, for instance, on high definition yarns such as cotton and cashmere.) But it depends on both how fussy you are and how even your stitches are in the first place -- as Elizabeth Zimmerman once wrote, a few good washes and wears and, voila, no more differences, not even to a very discerning eye.