The Portland shops have a fairly limited manufacturing shopfloor compared to ILW’s actual locomotive works and in 1999 had a pretty large pile of new locomotives in the pipeline as well as an increasingly large pile of repairs that needed to be done to the oldest class Bs (and the nearly-nonagenarian class As), so when the PV&T’s operations department ordered another 4 class K locomotives the shops decided to order 4 sleds from ILW, then install the drivetrain down in Portland.

The class K4 is almost mechanically identical to the class K units (it has a slightly higher horsepower and tractive effort, and it is officially a single-ended machine instead of being equipped with a boxcab on the tail), but it uses the streamlined design that ILW was starting to use on passenger locomotive orders (a design that was inspired by GE’s UM20 way back in 1953.)