I play two 4-Valve Flugelhorns, a Getzen and a Kanstul. For the flugelhorn, the 4th valve provides better intonation for 1,3 and 1,2,3 valve configurations all such combinations starting from the first D below staff. It also eases the bridge between lower register and pedal tone and does get me to harmonics that I could not reach easily on a 3 valve flugelhorn. My experience is unique however to a flugelhorn, not a trumpet.
I have never played a 4 valve trumpet and not sure the upper range argument would be reasoning for me to have such a horn after watching the Stomvi's 4 valve video. The double high C played by the 4 valve trumpet on the last note is performed with no valves used (standard key use). So I am not sure why they would make such a claim. I have not trouble incorporating the double high C on my Harrelson, Martin Committee and Olds Ambassador trumpets, all with 3 valves.
Again, I love the 4 valve feature on the flugelhorn for intonation and lower note bridging features. Also hate the idea of playing the fourth valve with the index finger of the left hand. Many people use 4 valve horns this way. I like keeping the right hand in charge of all the valve work so have no pinky finger ring on the 4 valve Getzen (which is the standard issue for that horn) and had Kanstul not install the pinky ring ring on their flugelhorn (which was their standard feature).