I dare to disagree a bit. They are all grammatical, since as and than can be viewed as being either a conjunction or prepostion. After prepositions, you can only use a pronoun in the objective.
The use of "than" as a preposition is accepted by some and rejected by others. American Heritage Dictionary marks it as a "usage problem". "As" can be used as a preposition, but when it is used in a comparison "as...as", it is considered to be a conjunction, at least in formal English.