Bottom plate 2" wide, but need 4x4 post
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.