It was my understanding, from the distant past of 40 to 50 years ago that those narrower siding boards were not replacements for damaged siding but were originals. Supposedly, the "shops", when they were building no. 4/6, ran out of the wider planking but had the narrower on hand so they used it.
Allan Kreig's 1962 book about the West Side states that no. short no. 4 was "purchased from Swayne". If that is accurate, it would indicate that no.4 was built prior to 1940 when the WS bought the Swayne Shays and rolling stock and Swayne shut down. Kreig observes that no. 4 was built with "knotty pine"; the narrower siding boards do not appear to have any visible knots in them.
So maybe the "shops" actually were Swayne's in Oroville and no. 4 was built during the Great Depression at a time when Swayne's cash flow was failing?
Best regards, Hart Corbett