Hi,
On a 4000/6000/8000 you can do this with no problem at all. Yes, you will lose a tiny bit of signal, maybe 0.5db when all are plugged in? With nothing plugged into them you will of course lose no signal.
The amount of signal loss depend on exactly which outputs and inputs you are using, I'm assuming any of the 4000 balanced outputs and patching into a 4000 input or other equipment with an input impedance of 10k ohm or higher - the usual type of thing. (If you're patching into six 1960's transformer input valve compressors then very impressive, and it will look wonderful, but you might have problems!)
One exception I can think of: don't do this on the channel insert points, they are not properly buffered outputs and are only just capable of driving a single input.
Cheers,
Andy