There is no rule saying what kind of rod length to fish a specific type of fish. It just the matter the length you use is the best for your the technique you use or not. If you plan to buy a high end fishing rod, they are technique specified such as jig & worm, frog, spinerbait or ....
And you also have to know where you going to fish. If you fish in water have a lot of grass, you need to use Medium Heavy power rod but if you fish in water have no grass area, even for a 4 LBS Bass, Medium Power rod is good enough.
Any length fishing rod can fish bass.