I need to hurry and go to the library; it will close soon.
Explanation:
A semicolon can be used to join two independent clauses when they are closely related in meaning. In the second option, a colon should be introducing what the sign says. In the third option, a comma should be separating the subordinate clause "Before it closes" from the main clause "I need to go to the library." In the fourth option, no punctuation should separate the proposition "at" from its object (3:00 p.m.).
