Another thought...
After thinking about my last post a little bit I realized I may have misunderstood your post. Perhaps the reason you want a boa under the size of 4 feet is not that you don't want a big snake, but because you would rather start out with a snake smaller than that, and you don't mind keeping a large 7 or 8 foot red tail boa. If you really don't mind a large snake like that, then of course a red tail boa is fine. But if, as I may have incorrectly assumed earlier, you don't want a snake that will ever be larger than 4 feet, you certainly don't want a red tail boa. As Cham keeper suggested, a ball python is probably a better choice for you. They are very similar in a lot of ways to a red tail boa, but they don't get as large. They stay around 4 or 5 feet long.
Another thought that I had was about why you want a free snake. Why do you want a free snake?
Do you want to adopt a snake from a poor situation from somebody that can't take care of it anymore because you want to give it a good home and are capable of providing it with everything it could possibly need - including veterinary care? If you do, and are willing to give it all the care it could need - then good luck! There are many reptile rescue organizations out there with red tail boas and ball pythons both that need both good homes and veterinary care.
But if you want to adopt a snake because you can't afford to pay a purchase price for one, then maybe you should rethink getting a snake in the first place. They can be very expensive to own, especially if you are to adopt one. Many snakes up for adoption have medical problems that their owners are unwilling to or unable to treat. That can become very costly, as their medical conditions have usually deteriorated to the point at which it is very expensive to treat. With my experience, usually free animals have these sorts of medical problems. Even if you were to get a snake with no medical problems, they can be very expensive to own. To start off with, housing them can be very expensive. For a red tail boa, they must have cages at least 4 feet in length, which either store bought or hand built can be very expensive. Either way you go you are probably going to spend at least a hundred dollars on a cage... at the very least. The snakes require special temperatures and humidity also. This can be a challange as they can counteract each other. If you use a clamp lamp with a heat bulb or ceramic heat emitter, this will quickly dry up the humidity in the air. If you use an under tank heater, or something similar, it usually won't dry up the air so badly. Neither of these heating supplies are too very expensive, and can last for years on end without needing to be replaced. The biggest recurring cost is going to be the food. These guys eat rodents, mostly rats or mice, but some will take gerbils and small rabbits. It is best to offer these foods pre-killed, as it is safest to the snake. So you can either buy them alive and kill them yourself, or buy them already dead and frozen. This can be very expensive, as even an adult ball python will even take several large rats a month. I don't know about rodent costs elsewhere, but here a large or jumbo rat can cost $8 - $9 each. And when you get to talking 4-8 of them per month, it can get quite expensive. You can also bet that a free snake in the size you're looking for will need some sort of veterinary care. They may have internal or external parasites, respiratory infections, or any number of diseases that will need to be treated as well, and veterinary bills can be very expensive.
For several years I helped a friend here locally who would take in free unwanted snakes. These snakes would come to him with all sorts of medical problems - from ticks and mites, infected rat and mouse bites, mouth rot, and even inclusive body disease. We spent a lot of money helping these snakes get well and caring for them around the clock, as well as money spent constantly getting check ups for our own reptiles to ensure they never were affected by any of the spreadable diseases and parasites that the rescued snakes came in with. In the five or six years I worked with him helping these snakes we not once got a good healthy snake that didn't need veterinary care of some sort. We even wound up having to put a handfull of them down as they were so sick or badly injured by the time the owner gave them up to us that they had to be euthanized.
So just a few things that you might want to consider before you actually get a snake... I hope this helps.