The referendum was 8 years ago but the Brexit took place 4 years ago
Some effects started before brexit was official. As institutions and individuals sought to minimize negative impact. I recall reading about european students leaving british universities pre brexit.
Depends on how you look at it. Some effects were probably visible even before the referendum just by announcing that. But on the other hand if you think “8 years ago is long enough that we should have seen all the effects by now” then the 4 year figure is probably more appropriate to make that evaluation.