His school ends at class 5. Move him at class 4, or let him finish and move at 6?

Our son's school only goes up to class 5, and he's in class 3 now. Nothing wrong with it, he's happy and his teachers are lovely. But he'll have to move at some point, and the question at home is when. My partner says let him finish class 5 with his friends and join somewhere new at class 6, when lots of children change anyway. My amma says the earlier the better, before the work gets serious. I can see both. Class 4 means leaving his friends two years early. Class 6 means starting fresh among children who have been together since nursery. If your child had to change around this age, which year was easier? 🙂