Completely honest and transparent answer: I'd keep/return based on how I liked the knot.
I'd feel justified that I should have a brush without such a defect, having spent what I had. If the knot wasn't to my liking, I'd request a replacement, and would hope the next specimen would have the knot shape I preferred. If I was lucky to receive a knot shape I liked, the former would still be true, but I'd suck it up knowing that knot shapes can vary quite a bit, and the replacement may have perfect engraving and a less favorable knot.
Make sense?
I'd keep it. If the handle was chipped or dinged or deeply scratched or something it'd go back like a flash but that's different to me. I would guess the reason they don't sell "seconds" like this at a reduced rate is because guys like me (assuming we were in the market for this brush at the time) would buy it for virtually no discount, because frankly that error doesn't affect the brushes value at all in my eyes. Assuming that JP is only referring to a visual inspection, not using the knot and then deciding, I could see myself following his method, but only if I really wasn't happy with the knot. If it were a cm higher loft than spec without some redeeming factor, then yeah I might return it "because of the 4".