I don't have evidence to back this up, but intuitively it seems to me that you might have a noticeable noise increase when going to a 4" barrel from a 5" barrel based on the powder in your cartridges.
A full load of a slow burning powder like H-110 may very well be still expanding violently when it exits a 4" muzzle. The extra length of a 5" barrel may (or may not) make an audible difference in such a case.
On the other hand, a load of a quick burner like Unique may have already reached peak pressure much earlier in either barrel and the noise difference would be negligible.
Just a couple of random thoughts with no empirical evidence to support them.