Why would the selling prices be low? I already have an offer of $800 cash for my 32 GB iphone 4 anytime.
It really depends on if the software unlock is available for the iPhne 4. If it's available next Spring, the selling prices for iPhone 4 will still be very high.
It's the 3GS prices that are dropping like a rock and will continue to drop.
The key thing is to dump the iPhone 4 about 1 month before the announcement of iPhone 5.
If you hang on afterwards, the prices will drop.