>>> " . . . but some people say they can't do 20mph in 4th gear".
I should hope not!
From the Institute of Advanced Motorists:
"Third gear is a very flexible gear in modern cars, including diesel cars, and it’s the ideal gear when you need to keep your speed to 30mph or under.
The IAM has a simple maxim. ‘Get the correct speed for the hazard – then get the correct gear for the speed’. At 30mph that gear should always be no higher than third!
In built up areas Advanced Drivers will often be in second gear at 30mph".
https://www.iam-bristol.org.uk/index.php/articles/associate-s-guide/27-gears-using-lower-gear-at-30mph
So, at just 20mph, a vehicle should either be in 2nd or 3rd gear and certainly not 4th!