Honestly, if the change is like the change from 2nd to 3rd, then NO.
If it's more like the shift between 3 and 3.5 then maybe. You see I've already invested a fair amount of money in 3 and 3.5 and would really hate for that investment to go to waste.
If 4th ED is drastically different than 3 -3.5 then NO I won't buy it. There are any number of other things that I can spend money on (video games, miniature games, comics, DVD's etc.). And a new edition doesnt stop me from continuing to play 3.5. If the well of 3.5 players dries up then that's when I stop playing D&D. No biggie, I've been playing role playing games since I was 12. I'm 35 now. There was a period where I stopped playing D&D (2nd Ed) because of rules creep and peoples facination with Dragonlance and FR. I played other games (DC HEROES, Mekton, Cyberpunk, Champions) until 3rd Ed came out. I REALLY liked 3rd ED so I started playing D&D again big time. If it's time to walk away I can say that it's been a good run.