What the frequency range means in vector network analyzers is that the device is capable of generating a pure tone up to 4 GHz or so, and it's internal transmission lines and parts are spec'd to not be too lossy up there. A VNA generates a pure signal of a known power, puts it out the port 1 and looks how much of that power (at that freq) is reflected back by the device attached to that port (and how much makes it to port 2 (vs frequency) if it's a two port device).
It is not doing any sampling or receiving really. And concepts like instantaneous bandwidth don't matter much.
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What the frequency range means in vector network analyzers is that the device is capable of generating a pure tone up to 4 GHz or so, and it's internal transmission lines and parts are spec'd to not be too lossy up there. A VNA generates a pure signal of a known power, puts it out the port 1 and looks how much of that power (at that freq) is reflected back by the device attached to that port (and how much makes it to port 2 (vs frequency) if it's a two port device).
It is not doing any sampling or receiving really. And concepts like instantaneous bandwidth don't matter much.