Not enough horns yet, Jonathan ?
I can´t comment on the PT4, because I don´t know that horn at all. I can state, though, that I´ve used the PT20 (B&S4097) in concert band with good results. Being the guy BEHIND the horn, I don´t have any idea how well it projects to an audience, neither. I WAS able to identify myself on a recording of last year´s world music contest in Kerkrade NL, though, playing in a 100 piece wind band in a section of seven tubas. (Just for the record: not all the time, and not because of wrong notes

)
Intonation on this particular horn is stellar. This horn is spot-on throughout the range and spoiled me for my beloved F-tuba, which requires much more work to play in-tune. Don´t know other specimen of this model, though.
It sounds larger than it looks, and its character can be altered successfully via choice of mouthpiece. To get the BIG sound I need for the large concert band, I personally need quite the toilet-bowl-MP and found myself looking for a piece larger than the PT50+ but with less edge to the tone than a PT88. The tone is smooth and rich, but I find it harder to alter sound colours (compared to the F). I don´t know if that´ll be the case with ANY contrabass tuba in comparison to a bass tuba, though.
As to ergonomics: this is a compact horn with the top bow close to the bell´s rim as opposed to the typical German style long-belled tubas. The valves are located so you´ll have to reach fairly high up to operate them.
This means that I prefer to hold the horn quite erect (not slanted at an angle to the left) so I won´t have to strain my right hand wrist that much. Because of that and the aforementioned compact design, the left hand will have to reach up well above my shoulder to get to the 4th slide. That took a little getting used to, and at first my left arm started tingling within minutes (no problem anymore).
The 4th slide is well accessible and can easily be pulled more than three inches (great for a well centered looow Db) with a flip of the wrist.
You´ll frequently have to empty the 3rd and 4th valve tubing, (remove slides and tip horn to the left) because they won´t drain into the main slide.
Will that help ?