Children take the grammar school entrance test at the start of Year 6. That means most parents tend to start preparing their children with mock tests and/or tuition about halfway through Year 5. So is Year 4 too soon to be thinking about the test?
There's no definitive answer. Every child is different, but it's probably not a good idea to pile on the pressure too soon. Having said that, many of the elements of the grammar school test are skills that would benefit from some practice anyway, so you could be doing a little work early on without making it obvious that it's grammar school preparation.
Reading a range of books helps develop comprehension skills and can increase vocabulary but try to read with your child so that they become familiar with thinking more about the text they are reading and looking up words they don't understand. Ask about what the characters in the story might be thinking or feeling - things that are not specifically mentioned in the text but can be inferred as this is often tested in comprehension tasks.
Learning times tables is really good preparation for numerical, or mathematical, tasks. Knowing what 7 x 8 is can make questions easier to answer and also reduces the need for lots of working out. This time saving can be crucial in the 11+ test, so being confident with times tables is probably the single and most straightforward practice that your child can do from a relatively early age. This skill is never a waste as being confident with mental maths will hold them in good stead for secondary school and beyond!