It depends on the child and their readiness for learning.
The course can be 1 or 2 years long, and I have many pupils who only start in Y5.
Some are not ready until Y5, whereas some benefit from the slower lead-in during Y4.
Year 4 = 2-year longer preparation time and slower lead-in.
Year 5 = 1-year preparation time with more intensive lessons.
There are advantages to both ways regarding the development of vocabulary, spelling, and mathematical knowledge.
Pupils will also develop their reasoning and application skills, as well as problem-solving capabilities.
I run initial assessments in the summer term before the course commences.
This also provides an opportunity for pupils to become familiar with the classroom as well as meet me and other class members.