Analog switches are a solid state alternative to relays, when you want a smaller, lower-power technology that won't wear out mechanically. And, as the name implies, you can use the Adafruit ADG729 Dual 1-to-4 Analog Matrix Switches to connect between two sets of four analog signals, much like 8 mechanical switches. These chips tend to be tiny surface mount parts, so this breakout will let anyone use the ADG729 switch for signals up to 5V, without fiddly soldering.

The ADG729 uses I2C to select which of the 8 channels switches to turn on or off. Four channels connect to the DA analog pin, and the other four channels connect to the DB analog pin. Note that unlike digital switches and multiplexers, these are not 'input' and 'output' because the signal is bidirectional. You could have the DA or DB signal be an input to 4 outputs, or the 4 inputs to one output.
Also, the ADG729 isn't really a 'selecting multiplexer', its a matrix switch with 8 independent switches. That means that yes, you could treat it like an DP4T where you select which signal is routed to the D pin, you can also turn on multiple switches to 'merge' the signals together. If you want all 8 signals to be able to route to a single pin, check out the ADG728.
Unlike a relay or mechanical switch, analog switches don't wear out, and the switch time is near instant, about 100nS. The ADG729 chip also guarantees break-before-make so the deselected switches will open before selected switches close.
However, there's a few things to watch out for:
The Vin power pin (the red wire if using a STEMMA QT cable) must as high as the highest analog voltages you want to switch. That means if the analog signals are no more than 5VDC, the V+ pin must be higher than 5V. You cannot power this pin with 3.3V and switch 5V signals.
It cannot switch signals below ground. No negative voltages can be applied to the Switch or DA/DB pins!
Analog switches are for signals, not power! Since this is not a mechanical switch, the signals pass through circuitry that is not designed to source or sink current. This is great for analog signal voltages, and is not good for providing more than a few mA of current.
In addition to the 8 switch S pins and the two D pins that can be switched to, there's also two I2C address pins so you can change the default address from 0x44 up to 0x47.
To get you going fast, we spun up a custom-made PCB in the STEMMA QT form factor, making it easy to interface with. The STEMMA QT connectors on either side are compatible with the SparkFun Qwiic I2C connectors. This allows you to make solderless connections between your development board and the ADG or to chain it with a wide range of other sensors and accessories using a compatible cable.