Punctually at 4pm and expect to leave by 5pm at the latest, if invited for tea.
But when they say tea time, they might actually mean dinner time, so I'd have to ask them for a more specific time.
However, "pop round at tea time" doesn't (imo) imply an invitation to either dinner or tea, though a few posters are assuming it does. It depends on the context.
Maybe they've just offered to lend you their Downton boxed set / punchbowl / second-best secateurs/ marmalade pan / skateboard. In that case, I'd expect the door to open as I/you approach and to be handed said item without being invited inside at all.
Anyway, I'd just ask "four o'clock?" And see what they said.
Am overthinking this topic, as a distraction exercise to avoid going up to the attic - no, not for Xmas decs! It's practically dusk now, and my excuse is that I don't like going up there in the dark 🤔 Procrastination accomplished. Thanks 😊