Pastorsoup doesn't mention cost or what, in his estimation, constitutes a four season bag.
For lightness, compressibility and reasonable cost, some folks sacrifice ulimate warmth, which might disqualify the following bag.
I know that bicycle tourers, who have an obsession with weight, use the following bag.
North Face Blue Kazoo sleeping bag
http://www.campmor.com/webapp/wcs/stores/servlet/ProductDisplay?memberId=12500226&productId=30682152
Some of them use it in this combination:
# North Face Lunarlight tent
# North Face Lunarlight footprint
# North Face Blue Kazoo sleeping bag
# Design Salt silk sleeping bag liner
http://www.backpackgeartest.org/rev...Silk Mummy liner/Owner report by Mara Factor/
# ThermaRest sleeping pad
I think a person could go down to a zero degree temperature bag that weighs 3.5 pounds, and easily spend over $250.
Getting those last few ounces off costs an inordinate amount of money.
L.L. Bean's has some surprisingly good deals on their Mt. Washington down bags and the Microlight tent.
Expedition stuff costs a lot of money and carries a weight penalty.
A three season not-so-serious bag (with liner and thermarest) and tent will serve a healthy, well-fed person in very cold weather.
Nonetheless, I look forward to seeing what some of our more serious backpackers have to offer in this thread.