If one person needs to change their travel plans, their booking can then be split off from the existing booking and managed separately. At that point, they would lose any benefits they have by being in the same booking (eg if they need your Silver status to pre-allocate seats), but the basic booking T&Cs would remain the same for them. Managing separately includes cancelling for any refund or for a voucher, if that's what's needed at that time.

in short, it's safe to make a single booking for all four of you at this time.