4-7 MPH will be noticeable. Because of this I don’t need a radar gun to tell how hard someone is throwing within a couple MPH. I can generally ballpark it from experience.
Visually, I'd say every MPH or so is noticeable, meaning if you watch a girl throwing 47 and another 49 it looks like a big difference (and it is over 40-43 feet) which leads people to thinking girls are pitching much faster than they are.