What is the acceptable margin for radiated emission on an European CE device
It's a limit, defined in CISPR 11 and other places. So anything below the limit is fine. If you're at the limit exactly it's a bit fuzzy situation, I can't remember off hand if the limit is up to and including the cut-off value. 

In any case, 4dB is a healthy margin, 2.8dB isn't anything to lose sleep over. If the margin is very small e.g. &lt;1dB you might fail at another time and different test lab, just because of how the cables are arranged, equipment calibration and so on. In most situations, if you're -0.1dB, they stamp the certificate and off you go. 

If you supply parts for a larger assembly, this attitude can come back to bite you when the client retests the entire setup. 

From your test limits, it seems you're testing Class B "normal" commercial or domestic environment device at 3 meter antenna position.