Tents provide wind protection but no insulation. In other words, a winter tent by itself will not keep you warm. Even if you had a yurt, with double walls and insulation between, it would have to be heated inside. Unless you're going to be where there are lots of snow and high winds, you do not need the weight and expense of a four-season tent. A four-season tent is designed primarily to hold up under winter blizzard conditions. I doubt you need these features in Florida. If you completely block off all openings in the tent, you'll end up with condensation inside which causes its own problems.

Your primary heat source, unless you have a fire or a stove (neither of which you want close to the tent for safety reasons) is always your own body. You therefore need sufficient insulation next to your body to keep the heat close! The extra insulation needed for cold temperatures comes from your sleep system and your clothing. You need a warmer sleeping bag, a warmer sleeping pad and warmer clothing. That's where you should be spending your money.

Other ways to keep warm are to force your body to generate more heat. This of course happens while you're doing vigorous exercise. You've probably noticed that while you're hiking in cold weather, you often have to remove layers to keep from sweating. (Do try to avoid sweating because you don't want your clothing to get damp!) Of course once you stop moving, pile on the layers immediately to prevent that extra heat you've generated from escaping.

A good jog/brisk walk/jumping jacks at bedtime will get your body generating more heat to help warm up your sleeping bag. Quite a few people also use a hot water bottle inside the sleeping bag (make sure it won't leak--this is one place where a Nalgene, heavy as it is, is useful). Chemical hand and foot warmers can also be used. Be careful that none of these are so hot as to cause burning, especially around young children.

Technically the process of keeping a constant body temperature under varying conditions is known as "thermoregulation." Here is an article that explains its relationship to backpacking, how your body loses heat and how to prevent this heat loss.