On the ground, it's just not numbered, so it's more "noref=yes".

If it wasn't shown and labelled on the official Sustrans map as being part of the NCN, it would actually make more sense.

Some of this unnumbered section of NCN is mapped online by EuroVelo as being their route for EV1 and EV2 rather than following the signed NCN 4... but that's probably more to do with EuroVelo not being up to date with how NCN4 seems to have gotten realigned to match a what seems to be a relatively new local route.