The sentence that contains a predicate adjective is the fourth one: "The weather today seems very humid to me".
A predicate adjective is an adjective that comes after a linking verb, that is, a verb that connects a subject to its predicate. In this sentence, the linking verb "seems" is connecting the subject to a predicate adjective ("humid"). Therefore, this adjective is modifying the subject of the sentence ("The weather today"). Predicate adjectives are opposite to attributive adjectives, which come before the verb and also before the noun that they are modifying.