The kiddos will be in upper berths overhead. It's tight but managable. The Fantasy has larger cabins than most ships with other lines, 186 sq ft. Most ships her age have far smaller cabins so if you're going to fit four, it's the right ship to do it with.
First, sorry for the late welcome but welcome to cruise-chat! You'll have to talk to your T/A to see what the availability is for your sailing. There are cabins that have a connecting door but they book very early. Cabins next door should not be a problem unless you're close to your sailing date. It'll cost a little more for two cabins but will give you the extra room you're looking for and with teenaged girls you'll be very thankful for the extra bathroom when it comes time to get ready for dinner!
Your poor hubby is out-numbered, I'm betting he'll like two cabins too. Poor guy will never get the bathroom if you all share a cabin!
It is still doable in a single cabin, it will be tight as Neil said. We have done it with 3 adults in an inside cabin on the Tropicale (smaller cabin). The thing we learned was to have a bathroom schedule.
You can still get two cabins even with the age thing. Just book hubby and one girl in a cabin and you and the other girl in the second cabin. You don't have to occupy them that way but have to do it for booking purposes.
Oops, didn't look back at the ship, my bad. I'm a wealth of mis-information lately. Thanks Dwayne, glad there are always some knowledgable folks like you to help when I go brain dead. Happening way too often lately, must be wave season!