Off to the Carribean next February for a cruise with 2 young teens and the spouse. Seeking fun activies for all, informality, no assigned seating at meals, accomodation tips (is there room for 4 in anything but an expensive suite?), any other tips re family cruising. Thanks to all.

This is the type of question that is perfect to pose to a travel agent. Maybe pick up the phone and call a local one, or get on the web and call one of the 800 numbers that pops up when you look around for cruise ships

Carnival sounds like it would be a good match. Informal, seems to be overflowing with teens and kids, etc.
There is assigned seating, but there is always the lido deck buffet if you don't wish to eat in the main dining room at assigned times or tables.
4 in a standard cabin might be tight, but you could always get 2 adjoining standards to get more room.
Carnival always has all kinds of activities going, so there should always be something for all to do.
Carnival has very reasonable fares most of the time.
I would check out Carnivals website, or maybe Royal Carribean.

Carnival is almost always going to give you the largest cabin at the lowest price, and your teens should like it.

And you might even be able to get a balcony cabin for you across the hall from an inside cabin for the kids for the same price as a suite on another line.