Now, after paying close attention, I see that in the video above (with 4 guys only) the one at the far right side is also a bass singer. Maybe the fifth singer sings the same musical line as the original bass singer and therefore there are only 4 distinct voices (1 first tenor, 1 second tenor, 1 baritone and two bass singers). If they stick to the barbershop harmonic pattern, they can be called a quartet.