Stop thinking about the maths and just think about the reality. You have 1 receiver. That means there is a signal coming from the receiver into the processor. There is only one signal. You can't measure it twice and get two different signals.
You may be thinking of the channel matrix, which has dimension 1x4 (not 4x1, sorry). That's because the channel ...
Multi-input single-output (MISO) is describing the inputs and outputs of the system. In communication, this is the channel, so if you have a single-output it means that the received vector is $1 \times 1$.
20Vrms is the maximum voltage you can apply across the transmitting transducer without the risk of immediately damaging it.
The amplitude of sound it produces is determined by the driving voltage. The transmitter is characterized at 10Vrms, so about 28Vp-p assuming a sine wave, probably where you would prefer to use it for reliability and long life.