Oversampling, decimation and Nyquist
Let's say I have an RF sampling ADC with a sample rate of 4GSPS. The device has built in decimation with a minimum decimation of 4X (and I/Q output, per comment below) so my maximum capture bandwidth is 1GHz (I know slightly less to allow for aliasing guard bands etc.).

Ultimately the ADC is sampling at 1Gsps with I/Q output, so it's producing $2\cdot10^9$ independent samples per second.  The Nyquist rate is twice the bandwidth, so between your analog filter and the ADC's, you'd need a final bandwidth of less than 1GHz.
You're asking if you can separate the signal digitally into a number of sub-bands and then decimate.  The answer is -- yes.  There's no reason that your filtering has to be in the analog domain -- you can just as easily filter in the digital domain.
So -- make a filter bank in digital-land, and decimate the filter outputs.  Done properly, this'll work just fine.
As an example, say you separate that 1GHz signal into 8 signals, spaced 125MHz apart and filtered to only include the middle 100MHz of each signal.  Then you decimate down to 125MHz.  Now you have 16 samples (8 channels, I &amp; Q) on a 125MHz interval.  That is the same number of independent samples per second that you started with, and you've reduced the signal content.  So -- you have less information, and just as many samples. You're fine.