4 in -- 1 out

[pacealot]

I'd like to second idy's suggestion of a rotary switch, as it could be totally passive (presuming that you only even need the one output) and even include a fifth mute position for when you change (which, given you're switching between four of them, sounds like it would be pretty often). But:

Back in my gigging days, I used to routinely switch between at least three basses (4-string, E-to-E 6-string, and 12-string — this is undoubtedly part of why I got fired and have never gigged since). I couldn't cope with the tangle of any more cables than I already had (stereo rig so one from the bass and two to the amps), so my solution was simply to hit a master mute switch, unplug bass A, swap, plug in bass B or C, unmute. But if you can manage four guitars with four separate cables without getting tied in knots, then go for it — more power to you! (Or, rather, less power, as in passive switching box...)

[doc_kelly]

I might just use 4 wireless units

I'd like to second idy's suggestion of a rotary switch, as it could be totally passive (presuming that you only even need the one output) and even include a fifth mute position for when you change (which, given you're switching between four of them, sounds like it would be pretty often). But:

Back in my gigging days, I used to routinely switch between at least three basses (4-string, E-to-E 6-string, and 12-string — this is undoubtedly part of why I got fired and have never gigged since). I couldn't cope with the tangle of any more cables than I already had (stereo rig so one from the bass and two to the amps), so my solution was simply to hit a master mute switch, unplug bass A, swap, plug in bass B or C, unmute. But if you can manage four guitars with four separate cables without getting tied in knots, then go for it — more power to you! (Or, rather, less power, as in passive switching box...)

[pacealot]

I might just use 4 wireless units

It's alright for some! 

[Processaurus]

Antonis: do you really want to offer 10k input-impedance for (each) guitar?

change them pots to 500k and remove the ground connection. will increase input impedance, and offer up to 16dB of volume reduction, should be enough i'd say.

cheers

The problem with solving the impedance problem with higher pot values is that the summing resistors to the opamp inverting input generally should be 10x the pot value, so the impedance doesn't change based on the pots setting or the taper get weird (maybe some other negative side effects?). 5 meg summing resistors would surely get noisy. A real Hi-Z buffer would perform so much better. $.75 for a quad TL074 and you're off to the races.