With model designations starting with the number 4, ALCO offered several four-axle variants of their Century series. The first model was the C420, powered by a 2,000-hp V12 engine and replacing the RS-32. By 1968, 131 had been built. The largest customers were Long Island Rail Road with 30 and Seaboard Air Line with 27. The second model that was also introduced in 1963 was the C424, delivering 2,400 hp from a V16. By 1967, 190 had been built. 92 of these had been built by Montreal Locomotive Works for used with Canadian National and Canadian Pacific. From the US-built locomotives, Nacionales de México was the largest customer with 45 locomotives.

In 1964, the C425 followed with an output increased by 100 hp. 91 had been completed in 1966 when production ceased. The Pennsylvania Railroad was the largest customer with 31. The most powerful variant was the C430 with 3,000 hp. Only 16 were built between 1966 and 1968, of which ten had been ordered by the New York Central. The more powerful engine of the C636 was not offered as a four-axle variant. Although the Century did never reach the numbers of its competitors from EMD and GE, many locomotives of the four-axle Century models were still in service in the early 21^th century.