You need to book for 4. Everybody, no matter how young, eating or not, needs to be included in the reservation because of fire code regulations. There is no guarantee that if you book for 3, that you can call and have the 4th person added. If there are no tables available, the phone cast member will not be able to modify your reservation. While castmembers at the restaurant will do their best to seat everyone and usually can, there is no guarantee and you will need to decide which person will eat elsewhere if your whole group cannot be accommodated.