Hi Michelle- if your reservation for 10 is at 1 PM in the afternoon on Thursday, cancel it. You will not have any trouble getting in most of the tents with only 4 persons and staying until 4 to 6 PM without a reservation. I suspect the price for your reservation for 10 is at least 250 € and you will get a Bier and half a chicken per person in addition to the seat reservation. The normal price for a Bier and half a chicken will be around 22 -24 € without a seat reservation. So paying the extra 150 € or so might not be worth it.
If you want to go around 4 to 6 and stay until closing it will be hard to get seats in some tents if the weather is good. The reserved tables have placards stating when the reservation starts. And not all tables in all tents are reserved every evening. But unless you get there by 2 PM or so most of these tables that are unreserved for the evening will be taken by folks who arrive at 2-3 PM and stay for most of the evening. And sometimes 1 or 2 people will claim a whole table at noon and keep it for their friends who arrive later. Sometimes the waitresses can get you an unreserved seat in the evening (Suggest a 10-20 € tip depending on the seat location).
You do realize that every tent (at least in my experience ) has an outdoor Bier Garten. And these seats are not reserved. And a lot of them have overhead heaters so you don't freeze if it is cold. Of course the smokers all sit outside since smoking is not allowed in the tents. But if the weather is very good (No rain , 65-70 degrees) try to get there by 4 PM on Thursday at the latest even to set outside. And don't forget the carnival rides...
Have a great trip!!!