While rough green snakes are easy to acquire and do primarily eat invertebrate prey, they rarely acclimate well in captivity, often heavily parasitized and usually refusing all foods. I don't recommend getting one as a starter snake.
Amazon tree boas can get longer than 4 ft, but they are very slender snakes and still don't require too much space as they rarely "stretch" out. However, they are very highstrung, often very snappy, and can be finicky eaters. If you're not looking for a snake to handle a lot, but makes a great display animal, ATBs may be the way to go.
Don't know much about flying snakes, but I do believe that they get longer than 4 ft. I've also never seen one offered anywhere for sale; in fact, I don't even think I've ever seen one in a zoo. Might be a reason for that...poor acclimation to captivity perhaps...