These trumpet quartets are written for beginner students. In our seven skill system, we call the absolute beginner level the Trumpet Pioneer level. These are trumpet pieces for students with a range only up to C in the staff. The key signatures are limited to one flat and one sharp. The rhythms are limited to quarter-notes, no eighth-notes or sixteenth-notes. There are other limitations applied, but those are the most obvious.
Trumpet quartets for this skill level are difficult to compose because the range prevents the proper voicings in many circumstances. If you are interested in trumpet ensemble music for absolute beginners (at the Trumpet Pioneer skill level), then you may consider looking at the beginner trumpet trios as well. There will be more of those just because they are easier to compose.