true 4" stock?

Outside of 495 there's The Woodery Lumber Company in Lunenberg, MA will definitely do it for you. A little closer F. D. Sterritt in Watertown carries 8/4 (say eight quarter stock when you're asking) which is a full 2" - you could glue together two pieces and sand/plane it to the exact size.

Many of the posts which are intended for a paint finish are made of many small pieces, glued together and milled to the finished dimension. Heck, I just recalled that our bedstead posts are made up of 16, 3/4" squares, stained, but still looks good. Laminations are inherintly more stable and less prone to splitting, etc.

While that is generally true it's a bit of a generalization. If the laminations are assembled without regard to the grain and different radial and tangential shrinkage coefficients of that particular species, the pieces will work against each other and they will delaminate over time. The bigger the piece, the bigger the problem would be