This fourth person would have to be on a separate ticket. I think you first need to figure out if these flights are sold out. I would go directly to the airlines' websites and check to see if you can book the same flights. If it's sold out, then there's not much anyone can do unless this fourth person wants to travel in economy instead.

You should also check the fare rules to see if you can apply the current ticket value to a new ticket on the same airline (you may or may not lose money if the new ticket is cheaper--again depends on the fare rules); refunds are only allowed for refundable tickets.

Also, why do you want to change your flights to GIG?