Tuning of the 4th valve on a Tuba

[iiipopes]

Yes, the convention is to tune the 4th valve as a perfect 4th below the open note. However, depending on the piece and the key, that may not always be the best:
1) For playing BBb in a lot of sharps, or a CC in a lot of flats, consider pulling 4 so 24 is in tune as a B nat or D flat, respectively, to avoid constant pulls otherwise
2) With a readily pullable 1st, and 3rd pulled to get 23 in tune, and no 5th valve, consider pulling 4 so 14 gets a perfect 5th down, and use 13 with a pulled 1st for the perfect 4th down
3) With a 5th valve, consider leaving 3 as the in tune minor third down because 12 is sharp, pull 5 so 35 is the perfect 4th, pull 4 so 24 is in tune, and the 45 will probably be close enough to a perfect 5th down.

There are others, and it finally depends on you, the horn, and what you want to do with it. If I played a CC regularly in a concert band or a brass band, I'd definitely consider #1. Granted, #2 isn't used much anymore because of the fifth long whole step now pretty much standard, and more esoteric variations such as #3 are probably too off the wall unless the piece really has a lot of quick movement in the lower register necessitating some alternate fingerings just to get it played.

There are probably even others, but the point is to be flexible. I did find some degree of irony that the author cited above, in his article about tuning a tuba, warns a player to be responsive and flexible with the individual tuning slides, then in the 4th valve article gets pedantic about THIS is the way to tune the 4th slide.

[Art Hovey]

The ideas expressed above by "iiipopes" seem to agree with the conclusions that I reached independently over the years. Here is my version, which says pretty much the same thing:
http://www.galvanizedjazz.com/tuba/fourthValve.html

I also find it very useful to have a second-valve slide that is easy to reach and pull when a passage requires both 4 and 24 fingerings. On two of my tubas I have a small bungee cord rigged to pull the 2nd slide back when it is released. Here's the junky one; the other is a Nirschl:

[iiipopes]

Hey Art - thanks for the post. Please tell us more about the customized tuba in the picture, especially how you offset the valves and valve buttons.