$4k used speakers will put you in another world sonically. Absolutely upgrade! Since you're relatively new to this, if you buy used I'd try to go with something that's not extreme one way or the other and that tends to shoot straight down the audio fairway. Some brands that many people like and rarely get negative comments (off the top of my head) are Joseph Audio, ProAc, and Silverline. There are a couple pairs of Joseph Audio Profiles (originally $7k/pr.) for sale here now in your price range, and I'd be shocked if they don't absolutely blow you away. If they don't you can likely sell them for little or no loss.
Couple last thoughts -- the Profiles (and many good speakers) appreciate some space between them and the rear wall to sound more balanced and image properly. Also, these speakers (and virtually all others recommended here) will greatly reward you with a good separate stereo amp and preamp rather than a receiver -- just a thought for down the road a bit, and something to look forward to actually. Best of luck!
Couple last thoughts -- the Profiles (and many good speakers) appreciate some space between them and the rear wall to sound more balanced and image properly. Also, these speakers (and virtually all others recommended here) will greatly reward you with a good separate stereo amp and preamp rather than a receiver -- just a thought for down the road a bit, and something to look forward to actually. Best of luck!