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.
Early S-4
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.
Late S-4
In 1955 ALCo and MLW began producing switchers with welded car-bodies (previously this had been done on cab units and road switchers [except RS-1s]). At this time MLW redesigned the car-body. The number-boards were moved from the sides of the car-body to the front of the long hood, there was a sharp feathered edge along the top of the nose and the sand filler lids were relocated from the roof to the front of the long hood. CN units were 8036-8079 and 8163-8195 and all units were fitted with external (exposed) rather than internal car-body filters.
However ALCo continued to build locomotives with internal filters while at the same time building some with external filters. GTW 8034-8035, 8082-8090 and CV 8080-8081 had external air filters while GTW 8196-8204 and GT 8205 had louvered doors with internal car-body filters. The number-boards and sand filler lids were not relocated as they were on the MLW units. The units were delivered in the black and yellow paint scheme with railroad gothic numbers and maple leaf monograms (instead of the wafer monograms that were used on the early S-4s) on the cab sides. After 1961 CN units began to appear in the black and red-orange scheme with the CN logo.On the US (ALCo built) units with external car-body filters, a smaller GT or CV logo was used because these external filters were positioned differently from those on Canadian (MLW built) units.
All units were retired by 1985 except GTW 8082, 8084 and 8199 which were rebuilt with Caterpillar engines to CS-9s 1000-2 and retired in 1992. CV 8081 was acquired by the Connecticut Eastern Railroad Museum at Willimantic, Connecticut.