Some of you can probably guess my opinion of 4th ed. Putting that aside for a moment here is my advice to deal with edition cycles. Personally I have been here before back in 95 when we went from BECM to 2nd ed, 2000,2003 and 2008. There are a few thngs I would and would not do.
1. There is nothing making you change if you don't want to. The WoTC ninjas are not going to come and take your books away.
2. Players will tend to try out a new edition and the old edition will not be very popular for a while as people flock to the latest book. Its new and shiny and old is bad. This can make getting new players harder but its not much of a problem if your existing players are not to fussed. Casuals will tend to play whatever the DM offers them.
3. Don't sell you books in the aftermath of a new edition and the time leading up to it. For starters you will not get a great price for your books as prices tend to be low and these days they will be getting dumped on Ebay. I am a hoarder myself and tend to buy older edition stuff if I see it somewhere and it is cheap enough. Use this time to complete any sets of books you may have missed.
4. Its probably a safer bet to not sell anything unless you are starving and really need money badly. In 1995 I sold off my copy of the D&D rules cyclopedia for $15 or so. I have been kicking myself for years over that one. If I sold it now I could buy alot of 3.5/4th ed/PF books but I still wouldn't sell it. Older edition stuff can still be usefull espicially a plot device/fluff or a map that you like. Things like that are mostly edition neutral. If you don't like it or don't want it pack it up and forget about it for a few years. Tastes change and you never know if you will want it again.
5. One doesn't have to buy a new PHB on release (or buy one at all). D&D is usually helped out by a few splats and the quality of a editions books tends to decline in the final 3rd or so of an editions life cycle with a few exceptions. Ealry splats in an edition cycle tend to be the good ones. The Complete Fighter, Complete Warriors and Martial/Divine Power are all useful books in 2nd/3rd/4th and all came out early in the edition cycle. PHB2 type books are good to get.
6. Aquire hard to aquire books sooner rather than later as the price will tend to go one way on these. Core rules, PHB2, MM and things like that tend to remain cheap forever. Limited release books and compendiums that are very useful to have and are popular tend to go up in price. As I understand it the Adventurers Vault is an expensive 4th ed book. IDK about the others but here are a few RPG books that I do know are expensive. Adventurers Vault, Rules Cyclopedia (BECM compendium), Pirates and Privateers (D6 Star Wars), Starships of the Galaxy and Knights of the Old Republic (Stare Wars Saga). The they are all very useful books to have in their respective genres. Its hard to predict what is going to be popular though.
Oh well thats more or less it.