The S-4 was simply an S-2 with AAR trucks instead of Blunt trucks. Like the S-2 it had a 1000 hp turbocharged 539 engine. However, in 1955 ALCo and MLW changed from riveted to welded car-bodies, and at the same time MLW repositioned the number boards and sand filler lids thus leading to two types of MLW S-4s designated early S-4s and late S-4s.
These had riveted car-bodies, internal car-body filters behind the louvered doors (where they were susceptible to oil and grease contamination) and were built up to 1955. All CN units were built by MLW and were numbered 7995-8014 (renumbered 8142-8161 in 1956) 8016-8025, 8028-8033 and 8142-8161. The GTW units were built by ALCo and numbered 8015 (originally assigned to the Central Vermont and later renumbered 8162) and 8026-8027. All were retired by 1985 except 8162 which was rebuilt with a Caterpillar engine to 1003 in 1981 and retired in 1992. By 1954 MLW introduced smaller cab side windows, as found on units 8028-8033 and subsequent switchers.