There is some basic information they will want to cover thoroughly, like most intake interviews but more of it. What else is included depends on the referral question. Sometimes it might be broken up into a couple of sessions, because four hours is a long time.

When I had an evaluation like that (not sure if it was 4 hours, but it seemed like it), I told a secret that I had been carrying around for a long time (a recovered repressed memory, actually - and I didn't tell all of it even, just that it was there), and after that I felt really good for 2 whole days.

Be honest and cooperative, and it might be a lot to sit through, but I think it can be very helpful.

Good luck with it!