My worry now is the appeal panel will focus on the 4A score for writing which will weaken our case.
Is the HT correct to almost discount the SAT results obtained from the CAT testing?
I think he is more likely to rely on the school's ongoing assessment, rather than on a one-off test (which may or may not be 100% reliable). However, it doesn't stop you pointing out to an appeal panel what the CAT score indicated for the future.
Besides, 4a is presumably what was being said months ago (September/October?). It would obviously be helpful if by the time of the appeal (May at the earliest?) your son is exceeding expectations at school!
Best not to pressure him about this, though, if he is prone to anxiety. (It's not unusual for boys to be less strong at Writing.)
I agree with KCG, both as regards keeping an eye on what progress is being made in Writing, and as regards submitting late evidence.
You don't even need to state your case at the moment - simply write "Details of case to follow