Going on the topics they have covered in year 4 at DD's schoool, I would say that they are definitely working at level 4b (possibly 4a towards the end of the year.) I don't think they are working at level 5 yet because there are various topics they haven't covered yet.
Here are a list of topics that I would 'think' are level 5, (I am going on a level 5 maths book that DD hasn't yet done)
averages (mode / median/mean/range)
missing angles on a straight line
irregular perimeter and are (ie like an L shape with not all the measurements given)
missing angles in a quadrilateral
(this question is taken from a level 5 maths book)
Find the value of the following expressions if x =18
(43-x) + 24 = ?
A lot of these topics are in the Bond 9-10 books (mixed in with easier questions of course - thankfully) and I would guess that Bond 9-10 therefore is a level 4a/5 book.
My target is to finish Bond 9-10 (book 1) before the end of year 4. Cover the 'new topics' over summer and then start the Bond 9-10 (book 2) in the first term of year 5. That's the master plan! I'll post back if we stick to to it