I wouldn't worry about other kids catching up if your son is HG+. A few months at age four will not radically change the testing norms. On the other hand, if you feel that your child is ready and willing to undergo testing, then there's no reason to wait. The sooner you have information in your hands, the sooner you can start making choices, right?
I have one child who shut down completely when she was tested at age 4.5. She cooperated for about 45 minutes and then just stopped answering. The test administrator was very professional, and she tried several different subtests to get her going again, as well as a snack break. Unfortunately, my daughter can be very stubborn, and when she decided she was finished with testing, that was that. We had PG results on the two subtests she completed, but that was it. When she went for assessment at age 6.5, she was mature enough to complete the assessment, and she qualified for DYS.
You know your child best, and I encourage you to follow your gut. In hindsight, I should have known that my introverted little girl wasn't going to be a willing participant at age four!
take care-
