Communication between two Arduinos over RF module
You have 4 channels of raw HIGH/LOW communication.  How you use them is up to you.

The simplest method is to use an asynchronous serial protocol:


Use Serial: Connect the TX of one Arduino to channel 1 at the TX, and RX channel 1 to RX on the other Arduino
Use SoftwareSerial: The same as above but you choose the pins
Use VirtualWire: very similar to SoftwareSerial


For higher speeds, since you have 4 channels, you could use a synchronous serial communication protocol. It should be perfectly possible to connect three of the channels to SPI and communicate as if it were talking directly to an SPI device - one channel to SCK, one to MOSI and one to SS. The other end has SCK, MOSI and SS but running in slave mode.

No need to "invent" a 4-bit protocol, it's too much work.