yes exactly. now the questions is if the 24-105 requires 1/4 of a second (compared to 1/2 on the f/2., the IS allows you to hand hold it more steady, so what is the "effective" shutter speed? People claim that IS compensates 3-4 stops. does this translates to directly an effective shutter speed?
Are you thinking that IS changes the shutter speed or ISO setting? It doesn't.
If the f/2.8 requires 1/2 sec at 2.8 then the 24-105 f/4 will require 1 sec at f/4. If the IS compensates 3 stops it would be like
trying to hold the camera at 1/8 sec. both lenses require the same amount of light to make the shot.
At a higher shutter speed you may move out of the "shake" zone and into an area where you can handhold better. If the scene requires 1/15 at f/2.8 at the ISO you picked it will require 1/8 at f/4 with either lens but holding a lens with 3 stop IS at 1/8 sec is like
holding a no IS lens at 1/60 sec because the lens reduces the vibration/movement AS IF the camera was shooting with a shutter speed 3 stops faster. 3 stops.