Canon EF 24-105 f/4 L question

[Sheiky]

Currently I have a Tamron XR di 17-50 f2.8 and a Canon 100-400 f4.5-5.6 L is. That leaves me with a 50 mm gap which sometimes results in a bit of frustration when I can not get the picture I want.

If you are thinking about replacing the Tamron with the Canon, you must consider that you are going to fill the gap from 50-100mm which sound like a lot. However you end up with a loss from 17-24mm. Which has quite an impact on your wide-angle capabilities.

So I thought about the 24-105 f4. Will this lens still provide me with good quality pictures at cloudy conditions with objects moving around 50 km/h (1:320 to 1:400 shutter speed) without going above 400 ISO (I find higher ISO settings on the 40D to give to much noise)Or is that too much to ask from this lens?

Your question is a bit unclear to me. You normally shoot at f2.8 with those settings and you ask if you can go up to f4 and bump your ISO one stop and get the same quality? Is that what you mean? A lens does not control noise, the camera does.