Hi Darcy and welcome to Cruise-Chat.
Sometimes rooms are available when you get onboard a ship, and sometimes not. If one is available, the price will depend on the type of room that's available. Considering you're cruising for free, getting another room will be considerably more expensive, no matter what kind of room it is! The least one is likely to pay for a 3-night cruise in a minimum category is about $300 per person, depending on the time of year you're traveling. If your cruise is longer than that, the price will be higher.
Four in a room can be crowded, but on the other hand, you don't spend a whole lot of time in your room other than sleeping and changing clothes.