A four day pass would have places to fill in four calendar days. You can use the pass for unlimited travel on any day that is filled in. So if you use it for 90 minutes in the morning, you can still use it for any travel you want in the afternoon. Or, you can make one long trip from one end of France starting in the morning to the other ending before midnight that night, or you could make 15 half hour trips all day long. It's for that day.
It used to be that to accommodate using a night train that started one evening and ended the following morning without using 2 days of your pass, night trains that started after 7 pm in the evening and reached their final destination after 4 am the next morning only required using the second day of the pass, but, with the loss of most night trains, that is hardly relevant today.
Another thing to keep in mind is that Europeans make numbers slightly different than ours. For instance, a European 'one' looks like a droopy seven, while they make a seven with a crossbar. Apparently someone once filled in the date, which was the 7th, with an American 7, and the conductor claimed it was from the 1st, and they had to use another pass day for that day.