4B is average for Y6 - so not at all bad.
All children are tracked throughout primary to ensure they make adequate progress. Children should make one level's progress every 2 years, roughly 2 sub-levels a year, but not many children progress in a nice neat line.
Formal targets are set for Y6 at the beginning of Y5 - they are challenging and realistic. It is in the school's interest to get as many as possible up to level 5.
I wouldn't worry about a target of 4B at all, as that means there are probably no real issues, just encourage enjoyment of learning.
If formal targets were set at the beginning of Y5 at DD's school why would neither child nor self be made aware of them? The targets we were given were the ones set at the end of Y2 - which she had already met.
4B may well be average for Y6 but here in elevenplusland average (sadly) doesn't tend to cut it.
If I were Scarlett I'd be keeping an eye on the school - and making the odd trip to Waterstones as well.
In fact - that's what I do anyway.