There is no answer based solely on the fact of some bike having 5" of travel. There are bikes with 5" I wouldn't take over anything more than 3 feet and some I would take off pretty much anything (although I've never hucked off anything over 6 feet, so...). I just took my Heckler off 4 footers several times today (I am 6'3" @250) and it's just groovy (the seat died today, but that's not the frame's fault). I ride with a couple guys that routinely take 6-7 footers on hardtails (one on a Surly Instigator(sp?) the other has a Chameleon).
The answer would depend more on the quality of the frame/build as well as the components the bike is built with than just the amount of travel. Assuming it's a quality bike with decent stuff hanging on it you *should* be able to run it off 4' drops all day long without any issues from the frame. If you have good skills (something I don't have much off!) you could add several feet to that and be fine. Enjoy the ride!