Yes...profit margins are still high on the iPhone 4, so looking at it in a basic sense of making instant money, yes, the value is there.
In that, if this upgrade can be used on the iPhone 5 upon release, i'd say there is just as much logic in waiting until then, since you intend (right now) to simply replace it with another iphone 4, so you really wont make anymore or less money selling that phone this summer.
Bottom line...will you make money now? Yes. But if it can be applied to the new iPhone this summer, i would simply use your iPhone 4 until then, and then make the move.