Hi Housemad:
Mumsnet learning pages have guidance on the progress through National Curriculum levels here: www.mumsnet.com/learning/assessment/progress-through-national-curriculum-levels
According to this a NC Level 3b by the end of Y4 would be considered the expected achievement for the notional (and non-existent) average child. So your DC was doing slightly better at end Y4.
What may be happening, and I hasten to add I'm an old cynic, is the rather common 'game' played by staff working to a target. Basically the new teacher reassesses the child and decides where they are working at - as a result, they usually make a fairly tough initial assessment down-grading performance. This means that they can show the two sub-levels of achievement in the academic year they taught your child fairly easily - because in fact by down-grading the performance they really only have to raise performance by one sub-level.
The flaw in this system is that the next teacher does not have to honour the performance evaluation of the previous teacher. From the perspective of staff who feel under a lot of pressure - it makes sense to slightly reduce pressure by underplaying performance evaluations for pupils at the start of the year, so achieving that first sub-level is frankly a doddle (they're actually there anyway, or nearly there). Once one member of staff starts this fiddle in a school, it's usually not long before others or all staff follow suit.
My advice housemad is to ask yourself (and/or research it a bit more) where you think your child should be at by this point. A good starting point - although I stress this is 'gold standard' and most likely your school won't be working to this - but Campaign for Real Education has prepared some very parent-friendly information on what a child should be covering and mastering in a given school year. Info here: www.cre.org.uk/primary_contents.html - just select the area of curriculum you're interested in. But please also consider your child's personality, attention span, work ethic and the level of support you are able to give (as compared to what you received).
If it's any consolation my DDs have had this kind of downgrading every year (now Y5 & Y3). I've had everything from we start each year fresh so although they were securely working at NC Level 3a and only need a few more things tick off on the APP grid to achieve 4c, we start each year anew - so I need to observe her performing these NC Level 3a tasks 3 times before I can move her to 4c (for example)...of Mrs X often is too soft, so I've retested everyone to satisfy myself of where they are at and we'll start from there....or we generally find Mrs PSBD that children lose a great deal of learning over the summer holiday, and often slip back. I just laugh now.
Do what you can at home. Try and ensure any homework sent home does get done. Try to keep tabs on what they're doing and what friends/ relatives with children of a similar age are doing. It's not competition - its a useful yardstick on what is possible in better circumstances. Finally help where you can - encourage reading/ learning opportunities. Try to insist they avoid shortcuts - and really do the work (e.g. DD2 loves to just write maths answers and not show the calculations - I'm evil and insist each calculation is shown).
HTH