Do you plan to lift with it or use it for a winch? If the latter, the extra foot is worth having. But then, why not just buy a real winch?
Wyeth-Scott More Power Puller with Wire Cable About the time that you need lift something 4 feet into the air, something's gone seriously wrong! If you need that extra 12" of height, expect it to lean, and you probably will end up with a permanent bow in the bar.
Many, if not most of my coworkers won't even keep one in the truck; there are too many things which can go wrong. Instead, they prefer a hydraulic floor jack, and some also carry chains and a come along like the one in my link above.
I have a 5 foot jack but don't see the advantage over a 4 foot, for the reasons listed above. I also carry one of the power pullers I linked above along with plenty of chain, plus a chain saw. As time goes on though I've gotten tired of using the above so my trucks are now equipped with another handy feature...
reverse!