I'm a little confused here, if you have pair of nice playing and looking BEOLAB 3000, why do you want BL 4000, sound is nearly the same.
If you have a BEOSOUND 3000 its another mather, BL 4000 plays good with that one, but if you want BEOVOX you will need an amplifier.
But if I had to choose BL 4000 or 6000, I would go for BL 8000 from E-bay, check the serial numbers, 8 digits 2 first is year code.
09- 19994, 10- 1995 15- 2000 and so on, prices in good condition $ 2.000 to 2.500 depending year and condition
Beosound 3000, BL 4000, BL 8000, BG 2404,BG 5000, BG CD50, Beocord 5000, BM 901, BM 2400, BM 4000, BV S45, BV 3702. There is nothing we cannot do, but a lot of things we don't want to do!!