"So I suppose my next question is if a school's policy is only to aim for NC L4 on Y6 SATs - are they coasting? "
Depends on the intake, and on the individual children within that intake.
For my children's school - very 'middle class' area, low FSM, EAL and SEN, well-educated parents, very stable cohort, great pre-school education accessed by virtually all the children - to aim for NC L4 in Year 6 would be coasting, in fact it would be holding many children back.
For schools like the one I teach in - deprived area, exceptionally high FSM, SEN and parental illiteracy, very high mobility, about a quarter of the children arrive with no early years education at all and in many cases pre-verbal - to aim for NC L4 in Year 6 is exceptionally stretching and requires many children to make way above expected progress in every single year of their education.
Even within those schools, you have to look at individual children - my children's school does have some individual children with SEN who will not make Level 4, and my school has some children who will achieve Level 5 or 6. A 'blanket' statement for the whole school is not a sensible way forward.