My personal experience.
Looking at graphs of gear spread as a function of ratios and revolutions/minute (hence, scientific data, not just "someone's wisdom")
5th gear in a 5-speed is essentially in line with 4th gear in a 4-speed.
True, there is a minute difference in primary gear ratio, but it's negligible, and puts the two top gears on par.
So the real difference is that in a 5-speed, 4th gear is really a "halfway" gear between 3rd and top.
I have a 4-speed, and after my chain drive conversion, which upped final drive from 2.94:1 (belt drive) to 2.88:1 (17/49 chain drive)
AND
a rear tire change from 140/80x15 to an unexpected 140/90x15 (which upped tire diameter a bit)
I now have my final drive ratio which is some 8% taller than previously.
Not much, you might think,
BUT
in the real world, 3rd gear is now my everyday downtown cruising gear, and I can use 4th gear (top) only when doing at least 75-80Km/h (50mph)
So... if I did have a 5-speed, I'd often be cruising in 4th gear instead of either straining 3rd at 4000rpm or riding 4th at 2500...
On a big single, 500rpm DO mean a lot, especially at lower revs.
Oh... and DO fit a tach...