I was actually thinking about the same. If you need to measure 4 channels in parallel for a detailed problem analysis, just check each measure point separately first on the first channel (other channels not used) in FFT mode, to check if there are frequencies above the Nyquist frequency. After having verified this for your 4 test points, it is safe to hook up the 4 channels to your test points, forget about exotic anti-aliasing filters and enjoy your 300 USD investment =)
Here is my understanding of the Rigol scope and its limitations: The Rigol DS1104Z has limitations in the filtering of the analog front end implying that you only have 25 MHz bandwidth in reality when using ALL 4 channels. When you treat the Rigol DS1104Z scope as a 25 MHz scope, and only use it for debugging designs up to 25 MHz, you can safely rely that the high frequencies that could exist in your design, are properly suppressed with the low-order low-pass filter in the analog front end of the Rigol scope.
But even when you are only debugging 25 MHz designs with this scope, you will still have the risk of higher frequencies right? I mean, even if you use it as a 25 MHz scope, there could be higher frequencies that give false readings? Or do I miss understand here?
Actually, regarding high frequencies and noise. If your design itself is within frequency limitations, any other frequencies in the signal path must come from interference and noise. But if the amplitude is very low, do they still impact a lot? Can someone shed a light on actual frequencies of interference signals and noise, their respective amplitude, and their impact? Can noise and interference beat the Nyquist frequency in terms of frequency and amplitude? =)
What about the other Rigol series, and their reliable bandwidth?
For the Rigol DS1104Z: ALL 4 channels, 25 MHz is reliable bandwidth.
For the Rigol DS2302A: ALL 2 channels, what is the reliable bandwidth?