As the others said- native snake possiblities would be most likely black racer, although they wouldnt be quite that big, or an eastern coachwhip- some are pretty dark, and they are fast too, or maybe even an indigo snake, if they range there.
None of those snakes can hurt you very bad, although they can make you bleed, but keep in mind florida is unfortunately known for released/escaped exotics, so nothings entirely impossible. Best to play it safe in florida, on the super rare chance that is was an exotic like a mamba or cobra, although most of those issues occur in south florida, and are super rare(other than burmese pythons which are getting more common).
If you are really certain about the size, and not exzagerating like most people do, than I cant imagine a native u.s. snake that stands 4 feet aff the ground while hauling butt. Even a large coachwhip or racer would only stand a foot or 2 off the ground when crawling. and usually only inches off the ground. I have found, however, that most people, when scared by a snake, exagerate the size by at least 50%, if not alot more(and say they are are positive about their measurements)- no offense meant- just my honest experience from "rescuing" people from snakes. For example, most "6 footers" turn out to be about 30 inches. Again- not saying you are full of it, just what my experience has been with others.
If you could get some good pics from a distance, someone can verify what it is.
Behavior wise- most likely a black racer.
Large reptile collection, study u.s. snakes very much.