It's all very confusing now. Schools use different methods to set targets and different ways of grading KS3.
I've just finished my PGCE (science) and my last placement school had letters for KS3 and they didn't have targets.
(Oh, and I spent yesterday 'tweaking' Y10 grades for reports because the targets are for end of Y11 and their average is pulled down by the tests they sat in Y9 and the whole thing is now fairly meaningless but looks better for SLT
)
My new school I'm not even sure yet, I've been told they mark on where there are for GCSEs now, but also use a bell curve, I'm really not sure how that works?
Anyway OP, if your DC had good SATs a target of 4 won't be for their actual GCSEs, so it's either the end of KS3 target, in which case it's fine, or where they should be now, which makes it very high, that's currently considered a GCSE pass.
Or it could be an arbitrary number unrelated to GCSEs, in which case there might be cause for concern.
Good look getting to the bottom of it.