You will all fit - but whether you'll be comfortable or whether all your stuff (clothes etc.) will fit is entirely up to your perception. Lots of people book quad-occupancy cabins and do very well with them.
Weather is variable all season in Alaska; it can be in the 50s or the 70s, raining or sunny. Expect all of the above and pack (light) accordingly. Layers are your best bet. A sweatshirt or sweater, windbreaker, jacket, perhaps a fleece vest, and rain gear will be essential items for everyone. A hat or earband and gloves may come in handy too.