Hi LindaD, and welcome to cruise-chat.
I don't think a 4-night cruise is too short, particularly as it will be part of a longer vacation for both of you. As budget is also a major consideration, keep in mind the charges above the cruise fare, such as (but not limited to) tips, drinks, souvenirs, fuel surcharges, travel insurance, and parking at the port.
On another note, I would encourage your friend to fly to Florida a day or two before the cruise, if she has not considered this already. This serves two purposes: first, giving a buffer in case of bad weather, and second, allowing you to board the ship as early as possible to maximize the time spent onboard. Of course, this needs to be factored into the budget, as well, but is worth every penny.