As it has been said, the fourth dimension is time.
You were wondering about seeing 4d? Well, this is a little hard to explain but...
Let's say there are these little creatures that only live/move/see in two dimensions. They live all their life like that, never seeing up or down, just left, right, forward, and back. If you were to pick one up, it would have no idea what was happening. If you put it back down it would still not know what happened, just that some crazy stuff went down but now it's over.
Well if you were to move through time like that, it'd be the same way. You'd see everything around you moving extremely quickly, and once you had stopped, you wouldn't know what happened, just like the creatures that lived in 2d, because your body isn't designed to sense time like that. You wouldn't know how far away you had moved, or how quickly.