Fatbananas...I thought you were in Kent? Are you trying to get your dc into superselective, or normal grammar? No point in scaring dc or yourself with book 5 just yet, after all they have not even completed year 5 curriculum, so things like ratios, percentages, proportion, in my ds' s school are studied this coming term, he only learnt multiplying and dividing decimals last half term, which features a lot in book 4.
I think kcg' s primary school is exceptional in their use of these books to a high level, and her boys are v v clever! Square roots are done to a minimum in year 5 and 6, my ds who is at grammar, had to decorate the inside of his maths book with square roots at the beginning of year 7, and areas of circles are generally year 7 and 8!
One thing I have been using from the books, is the terminology, there was a chat in a thread about this somewhere before. Awkward questions use: sum,product, total, derived, difference, etc which are a swine to learn...to be honest, they throw a lot of adults too, so it is worth using the c questions to get dc used to the variations that may be thrown at them.