You have a strange definition of fact and hard evidence, DaL.
"Fact: My wife sees multiple cases of under-assessment first hand in the class where she helps." - I am also an experienced teacher, but would never dream of presuming to give a thorough levelled assessment in children that I did not teach day in, day out. She cannot possibly assess those children across all the Numeracy attainment targets in the role she has undertaken.
Much of the numeracy curriculum at level 4 just would not have been taught to these children yet - and if it was, why would the teacher bother, if only to underassess them?
Take Data Handling (AT4), for example. Are you really telling me that your wife has seen, first hand, evidence of Y2 children working on probability, line graphs, range, mean, mode? Because I don't believe you.
I think what is more likely is that she saw them working at a level 4 in one or two aspects of number - which is fantastic, but does not a level 4 mathematician make, I'm afraid.
"Your conclusion: She doesn't know what she's talking about." I didn't say that.
"Fact: A local head admits in confidence that underassessment happens regularly."
Really? That's not what you said.
"Your conclusion: She's starting a rumour/she must be drunk" When did I say that? You presented the argument as hearsay, not hard fact from the Head of the school in question.
"Fact: My DS was under-assessed earlier this year by a wide margin.
Your conclusion: I must have another DS I don't know about." I didn't say that, either, I said your stories are conflicting - is your ds the one under assessed this year - in which case, why haven't you or your wife, using your knowledge of assessment, asked the school specifically which aspects of 4c there is no evidence for?
Or is your ds the child who has remained static for several years? It is very unclear.