On our last cruise my sisters and I experienced the same thing, but in reverse where the oldest 3 were in college and the youngest was only a teenager. Our situation is probably a little bit better than yours since its the college aged kids that tend to fall through the cracks a little, but I can offer the following advice.
Like previous posters have said, during the day there are plenty of "family" activities with no age restriction. These include hanging at the pool, bingo, the random games they have throughout the day, movies, etc. The four of us found plenty to do together that wasn't age segregated and we had a great time. Port days are no big deal since you'll probably be spending time together as a family or doing some sort of excursion. At
Castaway Cay there is the adult beach that the younger two girls can't go to, but I think the family beach is more fun anyway.
At night it gets a little trickier since the activities tend to be a little more separated by age group, but even then there are things they can do together. Depending on their personalities they may want to check out the teen club and the college club just to meet and hang out with people their age. This could be especially helpful for the oldest girl as she might want to see some of the adult shows (some of which are fantastic) or participate in some of the adult only activities but not see them with her parents or by herself. Unfortunately, the college club can hit or miss, but its worth a shot since you never know who's going to be on the cruise. Both of my experiences with it have been great.
Overall, I don't think anyone is going to feel left out. There is so much happening on a cruise that it's impossible to see and do everything, plus you make your own fun. I think they are going to have a great time.
