Ever wonder why 4 inches is so hard to picture? Just realized most people think it sounds way bigger than it actually is lol. So 4 inches compared to hand width is probably the easiest way to get it - literally just your palm laid flat. That's roughly 10.16 cm if you're metric. I always use a credit card reference too, since it's about 3.4 inches, so 4 inches is just slightly longer. Or think of a TV remote's button section - usually right around there. The wildest part? Most people imagine it bigger until they actually see it in real life. It's like when you finally measure something and go 'oh that's actually smaller than I thought.' Anyway, next time someone mentions 4 inches, just compare it to your hand instead of trying to do math in your head. Way easier to visualize that way.