This is playing something like 120Hz (open), 90Hz (ii), 103Hz (iii), and 77Hz (i). The approximate lengths are:

 mouth
   |
   | 13.5"
   |         29"
   +---------------------  i
   |
   | 8"
   |     7"
   +------------  ii
   |
   |
   | 20"
   |
   |

  iii

With a standard brass instrument you have a single tube with no branches. Valves or slides change the length of that tube, and all the notes you play have frequencies that are multiples of the tube's fundamental frequency. The fundamental has a wavelength of twice the length of the tube.

What's going on here?

(It should be possible to make a cheap brass instrument out of PVC with no valves, just branches and holes you cover, but I don't understand the physics involved.)