If you want your money to matter for the sound, put it into the speakers. With a budget of $4000, I would put half (or more) into the speakers. At that price point, I would seriously consider these:
http://www.magnepan.com/model_MG_16
I would then look for an amplifier capable of driving them, which, if they are like Magnepan in the past, are an honest 4 ohms (by "honest", I mean they do not have any nasty dips in the impedance going greatly below the nominal rating).
I personally would stay away from tubes, as they are costly and, in my opinion, are not as good as solid state. For advice regarding that, you might want to look at the amplifier section of this site and see arguments about tube amplifiers (you may have to use the "search" function to find them).
Obviously, you should audition as many different speakers as you can stand to audition, and as many types (e.g., ribbons, domes, horns, whatever) of speakers as you can stand to audition.