Unless you have photos of a demarcation between the underside of the wings and the fuselage, I think it's a mistake of the artist.
The B&W photos are very low contrast, don't seem to have too much detail or hints as to the color on the side. One has to assume it's the normal shade on most K-4s with that camo pattern of the time, unless one sees proof otherwise.
Meaning, it probably looked a lot more like THIS:
than THIS:
