Brother 4 was a high ranking Brother of the Church of Besainted Pelagius. It was an affiliation that got him killed however, assassinated along with Brother 4 by an invisible attacker.
References
Gillen, Kieron (w), Hector Sevilla (art). "A Ghost Story." In StarCraft: Frontline: Volume 2 (paperback binding), pp. 122-169. Tokyopop, January 1, 2009. ISBN 1427-80831-7