My DD didn't prep at all. Since I'm such a "type A", I tried in vain to find anything more than the booklet they send in the mail 2 weeks before the test (which has the same questions as the explore website has). At one time, I think they used to print a full-length practice test, but I couldn't find it.
Re: whether to test a 4th grader - depends on the kid. It is quite challenging for that age, and some kids might be upset that they can't answer all the questions. I think going through the practice questions might give you a good idea.
