It is for 7 voices because there are 3 "solo" parts (2 tenors and baritone) and then the 4 voice men's choir accompaniment: TTBB. So, shrinking this down to 4 voices would really take a great deal away from the arrangement. I do have it arranged for 3 solo voices (trombones in my arrangement) and tuba/euph. quartet (or ensemble: 4 parts) accompaniment.
Email me off board if you're interested in this arrangement. You could do it with your 4 voices and bring in some trombone soloists.
Out,
Mike Forbes