What do you shoot? Do you need the extra stop of speed? That's the big question. I had a 70-200 f/2.8 and sold it to buy the 70-200 f/4 L (non-IS) because I didn't need the extra stop of speed nor did I need IS (everything I shoot is from a tripod).

As for your other question, absolutely sell the 28-135. No reason to keep it that I can say. You'll have a great focal range completely covered.

Honestly, if it's a long way off (a year or more), I would just go with the 70-200 f/4 for now as lenses hold value really well and you can always sell it/upgrade in the future when you get to that point. The difference of that one stop is almost 2x the price, I believe.

Also, it depends on money (as always). Can you easily afford to drop the $$$ on the f/2.8? If you can, I might tell you to go ahead and pick it up as it's a fantastic lens. But if you're like me and have to scrape a bit to afford things, I'd hold off for now, the f/4 will be awesome and you'll love it.

I own the 28-135, the 24-70, and the 70-200 f2.8 IS. It really depends on what your doing and your budget. I shoot weddings professionally and would say if your going to do that, those two lenses are the required of the bunch. HOWEVER, if your not doing weddings now, theres no need to spend the $$ if your on a budget. Keep in mind that photography equipment keeps its value very well, and if you take good care of your stuff you can get close to or as much as you paid originally if you decide to upgrade later.

Glad we could help! I'm confident you'll love it.