I don’t think it’s shocking that you would have no velvet items. I’m more surprised by not knowing the word though.
People responded initially to the OP saying it’s not at all surprising that an entire class of circa 30 4 year olds did not know what velvet was, because it’s some kind of ancient relic that nobody wears/ owns/ mentions in conversation anymore.
It’s still pretty common - if not the original stiff ‘proper’ velvet then plenty of velvet- like clothing still abound, particularly around Christmas with which I think it’s particularly associated, due I suppose to being once seen as luxurious and fancy.
Besides that though, and where it might be more of a concern, is the lack of reading not knowing the word indicates. It is a word that is used very commonly to describe other things - ‘velvety soft‘, ‘velvet-like fur’ etc appear in all sorts of children’s books etc. Plus by year 4, children will have had a number of years of art, DT and even history lessons in which it may have been feasible for it to come up.
So yes, it’s somewhat surprising that not a single child in that class could make a reasonable guess at it, if that’s the case. Obviously it definitely depends on the context in which it came up, and the particular cohort of children. And of course every child (and adult) will have some gaps or some words they just never came across, it could just be a fluke that 30 children happened to have the same gap (and if they have English as an additional language for example, much less surprising).
I do agree with other posters who think vocabulary is important, and that children should be exposed to a wide range of words even if they have no immediate use for them.