For starters, BlueTooth 4.0 uses 50% less power then previous BlueTooth (but can go as low as 1/100 of power), so that keeping BlueTooth enabled will not require as much battery on your phone (or laptop) as it did before. Also, BlueTooth 4.0 is backwards compatible with previous BlueTooth, so a newer BlueTooth 4.0 device will work with older devices.
The key feature of BlueTooth 4.0 is its low-energy technology (or called BLE Technology). This lets device manufacturers replace proprietary sensor technology with BlueTooth. BlueTooth 4.0 will also find it's way into small sensors such as Nike+, health monitors, fitness equipment at gyms (or home), NFC and others. The range of BlueTooth 4.0 is much greater, up to 640 feet.
Since BlueTooth 4.0 radios are smaller and cheaper, it is expected that ALL phones will have BlueTooth 4.0 by the end of 2012.
Below is a list of all known devices that support BlueTooth 4.0 (as of 10/18/2011)
iPhone 4S
Mac Mini (thunderbolt version)
MacBook Air (thunderbolt version)
Dayton heart-rate strap