Making a Simple Audio Mixer
That will probably work well enough.  Most devices are going to output a average level of 0, and 4.7 k&Omega; is high enough that I don't think any device will have trouble with it.  This method will allow the other signals to back drive each output a bit, but at 4.7 k&Omega; that is unlikely to be a problem.

For extra security, you could put 2 &micro;F caps in series with each resistor to guarantee DC blocking.  2 &micro;F and 4.7 k&Omega; have a high pass rolloff frequency of 17 Hz, so won't cut into your audio.