The radio signals seem to be standard RC servo signals. Can you access the signal before it is divided into 4 separate streams. That is how they would be sent from the originating transmitter. That would allow you to read all the pulses on a single pin and would allow you to write your own interrupt code to measure them if pulseIn() isn't suitable. I wrote some code to identify model railway DCC signals and it relied on using a specific pin that triggers interrupts so that it would not be possible to use that technique for multiple signals. I think it could be amended for your purpose.
Perhaps you could multiplex the 4 pulses with a few logic gates - maybe they "accidentally" retain the channel spacing that would have been in the transmitted multiplexed signal.
...R