There is a difference between the
fourth spatial dimension and the
temporal dimension which some number as the fourth.
Someone who can perceive and manipulate the temporal dimension can prove it to you by disappearing and reappearing in a different place - in reality he simply goes forward in time and space at an increased rate so that to you it appears he has disappeared and then reappeared.
Someone who can perceive and manipulate the fourth spacial dimension can prove it to you by performing open heart surgery without opening anything. (Assuming, of course, he's a doctor -- otherwise you might be happier if he stayed away from your heart...) In the fourth dimension, someone can see and manipulate all three spacial dimensions that we are aware of. Take this Flatland analogy: a triangle trapped in a square can see no way out, but a sphere can simply grab the triangle and place him outside.
For good measure, you might want to find someone who can prove he truly occupies and perceives two dimensions by having him jump on a Mobiüs strip and returning with his heart in the wrong side of his chest. That would be cool. (Alternately a four-d person could do this with a Klein bottle).