Further to 4th Dan testing, in the last two years our Shihan has been testing rather than recommending 4 th Dan ranks.
I think as Peter suggested it has somewhat to do with cost since it is a lot cheaper.
As for the format again it is like Jun said where the test is about 30-40 minutes long and much the same as Nidan or Sandan.
6 ukes chosen by Sensei and mostly freestlye with some requested aspects such as tachi waza, hanmi handachi, suwari waza. He also asks for Jo dori, tachi dori and tanto dori.
One thing I have heard him say after tests is what he expects from the student. Ie: Shodan errors are still a lot of reflection on the teachers teaching.
At higher levels any faults become more the responsibility of the student and at Yondan the student should be able to figure stuff out himself. The teacher no longer feeds technique, etc to the student. Therefore if he see's any fundamental problems he will more likely think the issue is within your self rather than poor instruction.