(2) It's unfortunate [that we meet under these circumstances].
(3) I have it on good authority [that you are in charge here].
(4) It's for that reason [that she is currently number one].
In  the bracketed content clause is an extraposed subject. The dummy pronoun "it" serves as subject, and the that-clause as extraposed subject. Compare the basic, non-extraposed, version [That we meet under these circumstances] is unfortunate, where the bracketed content clause is subject.
 is also an extraposed construction, but this time it's not the subject that is extraposed, but an internal complement. Here the dummy "it" appears as object and the subordinate clause as extraposed object. The basic, non-extraposed version is inadmissible by virtue of having the subordinate clause located between the verb and another complement: we can't say * I have that you are in charge here on good authority.
 is trickier than the others. The bracketed element is a relative clause in an it-cleft construction. But unlike typical relative clauses, it isn't a dependent of "reason", i.e. it doesn't modify it. The words reason that she is currently number one do not form a syntactic constituent. It's for this reason that the relative clause is analysed as a postnucleus, not as a modifier.