I'm not sure Smith has the size, or more importantly, the lower body base, to be able to defend at the 4. I saw Smith live several weeks ago. He looked about 6'6" to me, and I'm more confident of that guess than I otherwise would be because I was with a friend who is almost 6'4" (just measured as part of a physical) and Smith was standing next to my friend at a break in the action. More to the point, he doesn't seem to have much thickness in the lower body. One thing our undersized 4s (Novak, GR III, Irvin, etc.) have had in common is sufficient base so that they don't get pushed off the low block. I'm not sure Smith has, or will develop, that base.
I like Smith, and would be happy to get him. He could certainly be a 3, and if he could develop his lower body sufficiently or grow height wise, he might be capable of being a 4. However, personally, I would want a clear 4 in the class along with him.