Two words of advice:
be flexible. If your main concern is whether or not you'll be able to find people to play with, then be flexible: don't be overly identified with any specific rules set, be willing to try new things, adapt to different situations and games. On the other hand, given the dearth of DMs relative to willing players, take one from a cheesy Kevin Costner movie: If you build it they will come; that is, if you say "I'm running a 4E game if anyone wants to play," chances are you'll be able to find folks, if for no other reason than you're someone willing to take on the time and responsibility to DM, so you should have a say in what you run. I always feel that the DM should have the lion's share as to what game is being played, considering they're investing the most time and money.
I miss just about every RPG product that I no longer have at least once in a while- even most of the real stinkers.
I can't tell you how many times I've sold and re-bought (and, once or twice, sold and re-bought
again) a gaming book. This goes for D&D editions, as well. A few years ago I sold off the vast majority of my collection, including all my 3.x books (I knew 4E was coming out). About two years ago I went on a total binge, buying tons of stuff, including a bunch of 3.x stuff. Thank the gods for Ebay (and various other online sites that I shall not mention)...
Back to the OP, I think it depends upon how you identify yourself, or rather what your relationship to the hobby is. Are you a gamer or a gamer/collector? Most diehard gamers--which include almost every regular on this site--are gamer/collectors; a major part of the run of the hobby is buying books, having bookshelves filled with the stuff, caressing them, occasionally browsing them for entertainment or ideas. Most serious/diehard gamers have tons of gaming material that they'll never actually play or use beyond occasional browsing and dust-collecting. And the real diehards even have whole rooms dedicated to gaming and would never even think about selling an RPG book. There's a fun thread somewhere in the annals of this site in which people post their game collection pictures...you wouldn't believe what some folks have.
The nice thing about 4E is that you don't need all that many books if you have a DDI subscription. I stopped buying most of the splat books long ago, only buying the theme and campaign books, and the most useful references. So to answer your question, you might want to try a middle way: sell most of it, but keep the essentials and the stuff you really like. And make sure you keep the books that you'd want to be able to read again even if you never play 4E again. I have most of the old 1E hardcovers, for instance, though I'm pretty sure I'll never play it again (although you never really know).
Oh yeah: be flexible. Did I say that?