I think it remains to be seen just how much power is saved for phones with BT4.0 hardware as opposed to BT3.0. I think it will be negligible.
When they say "low-power connectivity," that means it can connect to low-power devices like pedometers, heart and glucose monitors (Devices that use button cell batteries that use extremely low energy but can last years). Another example is this:
http://www.sltrib.com/sltrib/lifestyle/52804888-80/bluetooth-devices-smart-chip.html.csp
Casio of Japan has said it will introduce a watch in late December that’s Bluetooth Smart[4.0]. It will be able to link to a smartphone and alert the wearer to incoming emails and text messages by beeping and vibrating.
Sony Ericsson had a watch five years ago that used regular Bluetooth to do the same things the Casio watch will be able to do. But the Sony Ericsson watch weighed nearly half a pound because of its big, rechargeable battery, and it lasted only three weeks on a charge.
Another description of BT4:
http://gigaom.com/apple/the-secret-payload-of-the-iphone-4s-bluetooth-4-0/
What can Bluetooth 4.0 do for you?
Thus far, not much. Bluetooth 4.0 requires both sending and receiving devices to have the technology on board before it can really start showing benefits for users. As mentioned above, it works fine with devices using older versions of the spec, but it doesn't get to take advantage of any of its power-saving features with Bluetooth 3.0 or lower
I was hoping Galaxy Nexus would have Bluetooth 4.0, but now I don't really care. I do not plan to purchase any new Bluetooth 4.0 devices for the as long as I own the SGN anyway.