I would go for the exec room upgrade, even if you have to pay the extra surcharge for the fourth person.
First, the food benefit will save you a lot of money. Teenagers eat a lot, and breakfast costs more overseas at FS hotels (easily $20/person for a continental breakfast). The free minibar/snacks in the room is probably replenished every day. My guess is that housekeeping will look to see if the minibar needs to be filled, and someone else will come in to fill it up. The evening snacks in the lounge are also good (they tend to be better overseas for Marriott properties).
The internet savings are also greater overseas. My guess is that in room internet will cost around $20-25 at that property.
Get the Marriott Premier Visa card because you will earn 5 points for every dollar spent at Marriott along with a free room certificate every year (category 1-4 at signup and 1-5 for future years). The silver status gives you another 2 points per dollar at FS properties. Marriott points don't expire with account inactivity.