If the domain (lets call it X) is re-directing to the blogspot domain (lets call it Y) - and the PR of X is shown to be 4. It is likely that the PR of Y is actually 4 and the PR of X is 0. However, because X redirects to Y, Google shows the PR of the "end-point". When you buy this domain and remove the redirect, the PR will most likely disappear.
I generally don't look at PR while evaluating domains. I look at back links. Does this domain have quality back links? (not back links to blogspot - but to the domain) If not, then most likely the domain is worthless (unless its brandable)
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