Here are two relatively easy pieces for brass quartet consisting of two trumpets, tenor trombone, and bass trombone. No bass trombone? In this case, you can get around it with a tenor trombone.
The old plainsong melody O Come, O Come, Emmanuel, opens with each player in turn playing one of the four closing phrases of the tune. The brass players can be separated by some distance, such as at the four corners of the sanctuary.
The synthesizer then comes in with a droning pad in an open fifth. Immediately after, a drum enters playing a rhythm in 6/8 time. The brass players then play the melody in 2/4 time, playing the even eigth notes against the triplet of the drum. The drum helps hold the brass players in rhythm, again allowing them to be separated and still play as an ensemble.
In the other piece, the quartet plays two of the popular tunes for Away in a Manger (Murray, Fitzpatrick) in 4/4 time, just to make the pieces sound more interesting.
TWO CHRISTMAS BRASS QUARTETS, O Come, O Come, Emmanuel, arranged by Tom Kirkland for brass quartet consisting of two trumpets, tenor trombone, and bass trombone, with synthesizer, cymbal, and drum, and Away in a Manger for brass quartet consisting of two trumpets, tenor trombone, and bass trombone, comes in a pdf file of 1056K, with a six-page-page score, a three-page score score, nine instrumetnal parts, and a license page, twenty-one pages in all. Performance time should be about 2:30 for each.