You'd have found the answer more quickly by reading through the LR4 threads here already, but since there's no difference (except cost) with the teacher/ student version vs. the full version, you simply open your LR3.x catalog in LR4 (back up first if you wish) and LR4 will use that to create a new LR4 catalog, leaving your existing LR 3.x catalog untouched.
Mike.