My son tested at 4 as well (just a few months ago). I just told him that he would be going talk to a man who would play games with him and would want to see what he knows. I didn't go much more in depth than that. I sent him to school at his normal time on test day and checked him out right before the test.
My son has a very hard time focusing on anything for too long so I thought the test would be a disaster but he was having so much fun that he cooperated way more than he usually does.
Good Luck!
