For a 4ft fence then yes 5' T post would be the minimum.. however, I would go with 5.5' for a 4.5' fence.. 4' should be the minimum height for a horse fence.
As for the smooth wire, is it an actual smooth wire or is it twisted (barbLESS wire?) I would go with the barbless in this case. You'll also want to add a smooth hot wire that sticks out away from the fence.. I would put it on both sides for any fence separating horses to discourage fighting through the fence.
For extra safety, you can add two hot wires, one half way up the t-post and one at the top (horses can avoid the top hot wire and still paw the fence and get caught). I would also buy T-post caps. They are white and make the fence more visible as well as protect horses from the unforgiving edges of the t-post.
General t-post spacing is 8'-12' I think ours is 10ft. Don't bother with fence stays. They bend, move and create sharp edge not to mention it looks like crap.
I hope that helps!
~ Starline Stables ~