IS doesn't stop subject motion though! You need to think how you are using the lens. If you need to use the lens in lowish light, the extra stop is a huge bonus - 1 stop really can make all the difference in getting a blurry image or a sharp one.
Say you are shooting a sport like football or soccer. Subject moves fast and you need a shutter speed of around 1/500th to stop motion. In lower light you'll struggle to get that even with f2.8. f4 is too slow for this type of work. Also maybe you are shooting a wedding couple in a lowly it church. Say they are walking towards you and with f4 the fastest shutter you get is 1/30th! (pretty common for me). This is just too slow but with f2.8 I can get 1/60th and a sharp image. That extra stop could be crucial in many other circumstances.
Don't get me wrong the f4IS is a superb lens but the f2.8 IS is even better!
it's all about what you shoot. If you are using it for paid work, you need the best you can get. If just a hobby, look at your use and your cash available and make your choice. You may be able to sell the f4L IS later on at not much less than you paid if you find it too slow.
I should add there are times also however when the f4 IS is a better lens. It's small and light compared to the 2.8. You can get by without use of a tripod for subjects that are not moving (or are moving very slowly). You can be pretty creative with IS too.
It's a tough one. For me, I shoot weddings and need the f2.8 aperture. Even on this lens though, the IS is a huge bonus.
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