Three stops faster than f/4 is f/1.4.
The better way to think of it is that if a shot requires 1/240s at 0EV, with IS enabled, you could get away (potentially) with the same shot in 3EV at 1/30s.
The 24-105mm f/4L IS USM practically lived on my camera for eons. It's only been replaced with the 70-200mm f/2.8L USM as there's no room in the bag for the 70-200mm other than on the body.
http://users.tpg.com.au/octane2/shoot1.html
All shot with the 24-105mm f/4L IS USM and straight out of DPP.
H