My DS has had two Ados assessments. One at 4 and one at 9. There are different versions/modules they do depending on the age of the child and if they're pre/non verbal or verbal.
My DS enjoyed both that he had and the assessors were all lovely. They are very highly trained. I believe it's a difficult process becoming trained in delivering the Ados as what they're looking for can be quite subtle. There are usually 2 people doing the assessment and sometimes another asking you questions.
Some places will give you the outcome on the day, other times they may want time to think about it and speak to nursery/school. Occasionally they may want to go in and observe a child.
The idea of the Ados is that a DC shouldn't be overly prepared for it as they want to see how they react naturally to their questions or observations. As your Ds is 4, it will all be play based. I think in my DS's they blew bubbles together, acted out a tea party and then looked at a picture book. It was some years ago!
You may be asked to wait outside the room while the assessment is going on or you may be allowed to stay in the room. At my Ds's second assessment I could observe through a two-way mirror.
I was asked a LOT of questions - things like his early development (did he crawl? If so when?), how is his sleep? As your Ds is 4, most of it will probably be fairly fresh in your mind.
The assessments can take quite a long time. For our second one we were there half a day so I would take snacks.
I was very impressed by the reports I received after both Ados assessments. They really 'got' my Ds and could totally see his difficulties.