Consider the piccolo trumpet in Bb (like the normal trumpet in Bb but one octave higher). The first three valves lower the main tube by 2,1,3 semitones. The fourth one lowers the tube by a fourth (5 semitones) and would therefore be equivalent to pressing the first and third valve simultaneously.
I read somewhere that in some designs, the tubing of the fourth valve is actually an extension of the tubing of the third valve, implying that in that case, the fourth valve cannot be used without also using the third one.
Where can I find more information about the layout of the fourth valve? Which design is more common?
