I have torn out the ceiling in my guest room with the intention of converting to a cathedral ceiling by replacing the ridge board with a ridge beam + posts. Structural engineer says I need 4x4 posts, which is all fine. However, AFTER that, I tore the wall open and discovered that the bottom plate is a 2x4 on its side instead of lying flat and the studs are flat side, too, so the wall is 1.5" thick instead of 3.5". Can I just attach another 2x4 to 2x4 bottom plate to make it 4x4 and land the 4x4 post on it as planned?
You say, changing the ceiling in your "guest room". For such a small span (1 room) for the new proposed beam, I doubt if a 2x4 post would be a structural problem. However, there are many factors that could affect your decision: 1) Bearing load: The 2x4 post needs sufficient area to bear on. The end grain of the post will accept about 3 times the load of what the side grain of the Beam or plate will accept, so your structural engineer would need to calculate that for you, 2) Bending of new post: A post can fail by having too much load causing it to bend. This is a factor of width (skinniest direction) of post and length of post. This is called "slenderness ratio", and your structural engineer needs to calculate this, 3) Connectors: The beam needs to be connected to the post AND the post to the plate(s). Using 2 plates on edge, will move the nailing pattern into the plates out of standard position, and your structural engineer will need to calculate distance nails to edge of plate, etc. and your structural engineer will need to calculate that, (see where I'm going with this...)
There are other factors too, like over turning (rotation) of plates on edge, hold down of plate to floor, etc. it's just not practical to calculate all the variables, that's why your structural engineer used the standard 4x4 post, connectors, etc.
So, why try to "hide" the post in the wall? Why not "expose" the post by letting it extend beyond the face of the wall and stain (or paint) it the same as the beam? This will create a "feature" to your room. You can then use the standard 4x4 posts, connectors, etc. (You may need to change the base connector to fit around the base plate, but that's easy...there's about 10 gazillion standard connectors to choose from.)
Remember, that 4x4 post has a load that needs to transfer all the way down to a footing, (or beam/joists). It can't just sit on the subfloor. Make sure you look under the 2x4 plate to make sure something will support the new 4x4 post.
It is really an easy fix. Get some 1/2" plywood ripe it to 3 1/2". Take some Liquid Nail or F-26 adhesive glue the plywood between the existing plate and another 2x4 and either nail or screw it all together. This will make plate 3 1/2" wide so that everything will be flush. This will also allow you to flush out any other areas of the wall where the you don't need a 4x4 with 2x4's. Also make sure the top plate is correct if not use the same solution. I would also consider using a 4x6 if you have the room because it's less likely to twist or warp and it will carry heavier load.