I'd refer to the Big Book of Voodoo
The 4th dimension is time, or so we're told - taking into account that 2D and 3D are all a matter of how many directions something can move, I think it's probably fair to say that if someone figured out how to time travel (I'm going with the faster-than-the-speed-of-light theory), it wouldn't be limited to one direction i.e. they'd be able to travel in the past or future or whichever way's their fancy, but as it was mentioned, because of the whole paradox thing maybe the the effects of traveling back would never even be felt?
I once read a story about someone who could travel in time and he kept bringing his past self into the present but because the past him was now in the present/future, the present/future him ceased to exist so he simply disappeared when he brought the kid to his time with him. Then the kid would grow up and repeat this whole cycle a couple more times before he realized why he kept disappearing when it happened. :P