Send multiple analog signals to PC audio input
Getting a little off the question here but I think we have an XY problem situation here.

Going by the comments you don't need to get 4 analog signals into the audio input of a PC.
What you need to do is get 4 analog signals into a PC/phone as cheaply as possible.

That is a very different problem.

For the price of a 4 input analog multiplexer and the electronics to switch between the inputs at the appropriate rate you could probably get a small micro controller with 4 analog inputs. This would allow you to sample all 4 signals constantly at a sufficient rate and then transfer that data to the PC/phone over USB.
It has the added advantage that unlike the microphone socket the USB socket would also be able to supply plenty of power to run the external electronics.