You need to multiplex the data prior to transmission,
and demultiplex the data afterwards.
that "multiplex" means is "combine the signals into
a single signal, in a way that they can be separated
later". "Demultiplex" is just the separation afterwards.
Since it's physiological data, I assume that
it's limited bandwidth. One way to send that kind
of data is to modulate an audio subcarrier for each
of your four signals. The modulation could be either
AM or FM. Each subcarrier would be at a different
frequency. For example, you might place a subcarrier
at 500 Hz, 1kHz, 1.5 kHz, and 2 kHz. Traditionally,
FM is used. The subcarriers are then added, and
applied to the transmitter.
might be done these days with a small microprocessor
such as the TI MSP-430 or one of the Atmel ATMini,
ATMega, or AT32. In earlier days I might have used
a 555 timer chip and a low-pass filter, but that
would not give as good performance.
receive end, the output from your single-channel
receiver would feed 4 bandpass filters (to separate
out each of the subcarriers), and 4 demodulators.
Filtering and demodulation would be easiest
with a DSP chip - TI's "Piccolo" might be a good
choice. TI sells small and inexpensive demo systems,
along with a complete development software.
In the "really expensive" category, IRIG telemetry
equipment might be used - Google it - but that's
probably out of budget range.