If you can find a clean pair, the Dynaudio 3.3's are actually quite good speakers, and they go for roughly $2,500-$3,000 used. They are also less forward than the C1/C2/C4, which is what many people find off putting about the sound of those models.
Of course, this recommendation assumes that you like the Dynaudio house sound, which you must if you like the C4's.
Of course, this recommendation assumes that you like the Dynaudio house sound, which you must if you like the C4's.