After the Southern Pacific had already converted several 2-8-8-2 Mallets into Cab Forwards, the 1928 AC-4 was the first class to be built as a cab forward from the start. Superintendent George McCormick was responsible and production took place at Baldwin. Like the AC-1 to AC-3, the AC-4 also had four cylinders of the same size without compound working. Thanks to a two-axle leading bogie, a larger firebox could now be accommodated. The delivery of oil to the front of the locomotive was ensured using the proven system with an overpressure of 5 psi. 

The AC-4's valve gear had a limited cutoff, which initially reduced the starting tractive effort somewhat and was later removed. After ten AC-4s, 16 AC-5s were built, which were slightly heavier. The 25 AC-6s, which also had a higher boiler pressure and a smaller superheater, were once again significantly heavier. They received a new, cast frame in 1947 and 1948, which further increased the weight. All three classes were retired between 1953 and 1956.