Hi Lydiasmommy,
We had our daughter tested when she was just over 4 for school selection purposes. My daughter did the SB5 and for the first 3/4 really enjoyed herself from the bit I could overhear. My daughter is also shy and slow to warm up to people, but I did choose a tester very experienced with gifted kids and I think that made a difference.
What we did find though was that at the 3/4 mark, she hit the wall with test fatigue, with 2 subtests to go. She had either reached the ceiling (and in some cases could have gone further if the test allowed) or was a point off for all her subtests to this point. Then she just stopped cooperating (I heard her yell "I sharn't!" - nice!) and the scores on her last two subtests dropped substantially. She still scored very highly and was one point off DYS level but the tester believes her results underestimate her ability because of what occurred. I think had we left it even a few months more dd would have had more stamina. I guess what I am saying is if you do need to test early, still try and leave it as late as you can (and that with a good tester shyness hopefully wont be an issue!)