I have had the experience of putting one of those 4-valve sousas on my shoulder, and playing a few notes. It seemed so, so heavy, about like having two King sousas on your shoulder at the same time. I don't know whether the weight was primarily from the 4th valve and tubing or the Chinese sheet metal (or probably both). It would be interesting to see how they fare in actual use.
I used one of those sousaphones for an entire marching season and it was probably the most god-awful sousaphone I have ever played. Not only is it hereby as sin but is extremely stuffy and very flat specifically in the 4th valve which is the only reason I could see anyone wanting one but it is so flat you might as well hack it off to save your shoulder from the weight.
I am used to the weight of a 4 valve - I have a Conn from 1929. It is quite heavy but I sure do like the extra valve for fingering speed and lower range (but the horn is also sweet in the high register!) I did have to have one part of the 4th valve pipe reshaped a bit - the pressure on my diaphragm was too much to deal with...