How many days are you going for that you would get non-expiration on a 4 day ticket? Just make sure that the left over non-expiring days will fully cover the future trip you plan on using them on, because in most cases it would be cheaper to skip the NE option and just buy all new tickets for the next trip if the remaining days don't fully cover a future trip. It really depends on how many days each trip is to be able to figure that out.