From a lawyer's point of view, you must check every two hours, but what if you had dropped under 4? If you are driving along when you discover this, because your partner has swiped for you, you would technically be driving illegally as from the moment you discovered this, until you could safely pull over. (Though if you were still driving and you’d dropped under 4, it could be argued that your hypo awareness was questionable!)
Legal nit-picking aside, we always aim to stop and have a coffee, loo stop, and swap drivers every two hours, just for the safety of being refreshed.