If it's possible to apply at 4 (not sure it is?), I'd make the decision based on what you think you will be able to achieve through DYS in terms of advocacy - because I think that's the piece you're looking for? Whether or not you'll get help that makes a difference through DYS, you'll want to be sure you know as much as you can about available options in my local area for whatever type of academic situation/acceleration/challenge etc you want for your child - so I would work through that first before worrying about applying to DYS.
OTOH, I wouldn't turn down an opportunity for free testing through the school district. It's a data point, it's free, it might not be as timely as you like, but definitely go ahead and do it (unless you feel you really really need to test sooner). If you decide to test privately, this is jmo, but I'd probably wait for a WISC (otoh, will you be able to get the WISC through your school district when your dd is old enough?). I've found that the WISC carries a bit more weight than the WPPSI in advocating - and with scores like you've posted above, chances are you are going to be advocating for most of your child's school career, so if funds and opportunities for testing are limited, I'd want to wait for the test that might carry a bit more weight and be less likely to be questioned.
One other thought - the school psych has admitted he's rarely seen scores that high - is there anything you can do to leverage his knowledge of how unique your dd's' scores are? Did he have any suggestions for how to place/accommodate your dd in her current school or in the school district? Sometimes you'll run head-on into brick walls with school staff, but if you keep chipping away with requests and you have data (which you have) and a school staff member who believes that your child needs what you're requesting (which it sounds like this psych may be supportive) then you can make progress - it just takes time and repeated effort.
Hang in there! And good luck
polarbear
