The two are intended for different use cases, though. Bluetooth 3.0 supports speeds of up to 24Mbps and is aimed at applications such as audio streaming and file transfers. On the other hand, version 4.0 technically includes similar functionality in its specification, but supports very short data packets of 8-27 bytes at a speed of 1Mbps and can operate on coin cells -- the kinds you find in wristwatches, calculators, and remote controls.
Such devices should be able to run for years while offering Bluetooth connectivity. In a dual-mode implementation, it is also possible to combine Bluetooth versions 3.0 and 4.0 in devices such as mobile phones, where high data rates can be achieved for certain tasks and low-power functionality for others.