tier 3, tier 4...schools go up and down (just look at syracuse). i think chapman has the potential and the ambition to be in tier 2 (some schools like albany law seems like they don't really care...they have been in tier 3 forever) or else they wouldn't invest so much $ into the school and its facilities.
also whitter doesn't have an undergrad so chapman gives you more resources.
edited: totally agree with dnw2007. personally, i think whether you graduate from a tier 1 or 4 will affect your job search initially. but down the road, its your abilities and knowledge that will matter. maybe a student from tier 4 will take longer to land a big firm job, but if you are good, you will eventually get one. however, even if you have a jd from harvard and you don't know jack, you can get hired by a big firm initially because of the institution that grant you the degree, but eventually, you will be weed out.