Hi, there. I'm happy to give my 2 cents on this.

First of all, yes, all four of those are grammatically correct --- all four are error-free.
Now, the more pertinent question -- how would the GMAT compare them relative to each other. Remember, GMAT SC is based on three criteria:
(a) grammar
(b) unambiguity
(c) directness & concision
All four pass the grammar check with flying colors. None of the four of them has any ambiguity: they are all perfectly clear.
The third criterion, though, does distinguish among them.
The second, "it is required that he attend the court briefing" is indirect and passive --- yuck. The GMAT does not like that at all.
The first, "he is required to attend the court briefing" is passive, generally not preferred, although it may be necessary for the overall structure of the sentence, e.g. "The oafish misanthrope who punched a crossing guard is now required to attend the court briefing about the incident."
The latter two are both active, both 100% perfectly acceptable. If the GMAT SC were asking you to distinguish between the two of them, it could only be through something like parallelism ---- for example:
"After his seven bar fight,
the court required that he attend the court briefing and to do 200 hours of community service."
OK, that's a horrible sentence, but the idea is: while the underlined clause is
completely acceptable in and of itself, it's
not in parallel with the second part of the sentence. The second part has an infinitive "to do", so the underlined part should be changed to something with an infinitive ---- for example, "the court required him to attend the court briefing." The infinitive "to attend" is now correctly in parallel with the infinitive "to do."
Does all this make sense? Please let me know if you have any questions on what I've said.
Mike