More than likely iPhone 5 will have dual core processors, a slightly larger screen, and a few other things that aren't that big of a deal to some people.
If these are things you would really like to have, then wait. If not, then i would get the iPhone 4. It's perfectly fine unless you really want a larger screen (which is not even a guarantee), and if you play games a lot to the point where a better processor is a must (which is also not guaranteed either).
If you're like me and really inpatient, then I would go with the iPhone 4. And who knows hows long it will take for them to release iPhone 5...