Hi

What is the difference in terms of sound between a wick 5BL and a 4BL

Thanks

A 4 BL is a little wider and deeper than a 5 BL. It would tend to make the lower register a little fuller and the high register a little more difficult, unless the 4 BL fits your face better. Note that I play a 4 BL because the 5 BL feels like "tight shoes" on my mouth, while a 4 AL feels like an air hog. If the 4 BL fits, it's a great mouthpiece. But just going from one to the other may not make things better; in fact it could make things worse.

You can even split the difference in size with the 4.5 AL. The slightly larger cup coupled with the much larger aperture may make it easier to put more air in the trombone if the B throat is backing up on you.

A mouthpiece change is indicated when you feel like you have hit a brick wall and your teacher has agreed that simply practicing more or smarter is not going to help enough.

I know that Denis Wick will allow you to try out various mouthpieces: they'll ship you two or three at a time so you can compare. I have a 4AL and 4BL, and overall, I prefer the 4BL as well (Besson 10-10, .555 bore).