When our family went it worked just fine with 4 in a room. Once you unpack a lot of luggage can slide under the bed and in the closet to make more room.
As the above poster stated Disney has some of the larger rooms in the indusrty , made with families in mind.
Now the newer ships the rooms are smaller from what i have read, but the Magic and Wonder are great.
Most rooms have a split bath. One room has the shower and sink the ohter the toilet and sink, both have mirrors too. So its much easier to change for dinner or someone can take a shower and if someone needs to use the toilet they can without interrupting the person in the shower. That seems to happen a lot with kids.
You will also have a couch and a chair at the desk which is plenty for 4 people. Most of the time your not all going to be in the room accept to change for dinner ro for the pool and to sleep.
As you can see by my signature I have cruised Disney many times and I have been in 3 different types of balconies, an ocean view and inside rooms and various decks.
I have also done Carnival. If I was taking 4 people I would pick Disneys rooms over Carnival anyday.
Good luck and have fun planning!!!