There is some information missing. If you demux 4 serial channels into one, you get garbage. Is there a serial protocol involved that identifies the 4 different sources? If not, there is nothing within the standard UART that would enable you to figure out which data is from which source.
As for the physical option to concentrate 4 serial interfaces into 1, you could use a MCU with 5 serial interfaces. There are some ARM9 devices (e.g. LPC3250) that can do this, there is a Cortex-M3 from Toshiba with "endless" serial interfaces.
If you are talking just 9600 Baud, you can use a small MCU and simulate the serial interfaces.
There are so many options that could all be feasible but with(out) your information it is not possible to provide a better answer.