One of the key features of Bluetooth 4.0 is its low-energy technology. With Bluetooth 4.0, accessories or devices that need to constantly on to transmit data to and fro the phone will benefit from it. Users won’t have to keep recharging their devices so often, and leaving them on for prolonged periods of time won’t dramatically affect battery life of the phone, and of the accessories (which can run off tiny batteries). Examples of such devices would be mostly in the health and fitness category: pedometers, heart rate straps, and blood glucose monitors.
Devices such as wireless headsets for listening to music or hands-free communications would also benefit from the low power technology. But as we all know, most devices now don’t even take advantage of Bluetooth 3.0, so it’s going to be awhile before we see Bluetooth 4.0 being widely adopted, but when the time comes, the iPhone 4S will be prepared to work with such devices.