They don't charge your card until you eat the meal, or do not show up for the meal, so it won't cost you any extra to make reservations for 7 as for 4. The issue is, you need to call 24 hours in advance to NOT be charged for no-shows, and if you call and tell them 3 other people won't be showing up, you're actually changing the whole table-size needed and they may not still honor that reservation.
I'd go ahead and make reservations for 7 people, and AS SOON as you find out whether or not they are coming, then change the reservation (if they have availability) to the right number of people. How far in advance will you know whether or not they are going to be able to make it?