Noise Bandwidth of multichannel system
I'm not sure that the noise bandwidth is what you need.

To simplify sums, assume that both the signals and noise are flat with frequency.

Given a certain noise density, like -174dBm/rtHz + noise figure, you will have 4 times the noise power in an 80MHz bandwidth, than you will have in a 20Mhz bandwidth. However, you will also have 4 times the signal power in 80 rather than 20MHz bandwidth. So when you decode any channel, you will have the same SNR, the same Eb/N0, which is the important parameter for decoding the modulation.

You might worry about noise bandwidth when computing the total power or peak voltage of a noise-like (that is, adds as power) signal, for either amplifier, mixer or ADC operating levels. As the noise power floor will generally be below the signal power when the system is working, you will not be worried about signal handling for for the noise itself. 

There will be practical signal handling reasons for using a preselector, and choosing to ADC all four channels, or each channel, to make a high quality receiver, but that will only affect the Eb/N0 incidentally, not fundamentally.