Although they explain on their website how they work out the serial numbers for their firearms, I do not know how it works for air rifles but it could be that the 4 represents 1974 and 06, June of that year. This is only a guess and probably wrong. It also begs the question as to how they then coded their 335s in the 1980s. Furthermore, quite possibly their air rifles all share the same coding method and the model is not identified via the code, although that would be surprising.
I was tempted to suggest to them that they produce a new and updated 335, now that Walther has withdrawn its LGVs and LGUs! That might have produced an interesting response.
Rgds
A