Many people do not know their birth hour, or only know a rough morning or afternoon. That does not make a compatibility reading impossible; it makes hour-related observations more conservative. Marking the limit honestly is better than inventing an hour.
What three pillars can already show
The year, month, and day pillars can show the day master, spouse palace, some ten-god relationships, and element distribution. They also support a basic comparison of interaction rhythms — enough for an initial understanding.
What becomes more conservative
Details tied directly to the hour pillar lack evidence. The system marks “birth hour unknown,” does not assume an hour, and should not present conclusions from missing data as certain.
You can add it later
If you later learn the birth hour, you can add it and recalculate. Understanding what is already known is more useful than getting stuck on an uncertain hour.