Why will the samplerate drops down to 1GSa/s when using all channels?
This would mean that 800MHz is not possible with all 4 channels active.
Lets wait for international prices and see if Rigol wants to place the HDO4000 as competitor to SDS2000x HD. HDO1000 should have lower price i guess.
Only 500MHz Bandwidth @ 1M Ω
Bandwidth (-3 dB) @ 50 Ω : 800MHz
Bandwidth (-3 dB) @ 1M Ω : 500MHz
A better BW with 50 ohms mode suggsets that they may have actually different input amplifiers and not just an 50 Ohms resistor to ground, which is good.
Probes for the 1 M input impedance are usually limited in the BW anyway, rarely more than 300 MHz and even whan the input is no longer really high impedance at 500 MHz.
With a single ADC for all 4 inputs, or all ADCs combined to get the full 4 GSPS it is normal that the maximum sampling rate will go down to 1/4.
For the highest BW one would need active probes and not many would buy 4 active probes. I would still be nice to have 2x2 GSPS even of the 3rd channel is used as a trigger.
12 bit resolution for the ADC naturally does not come with very high sampling rate.
It was a disign decision (possibly due to the available ADCs) that some of the 4 channel DSOs can't combine all power to one channel and thus only have 2 steps like 1 or 2 GSPS depending on the number of channels used. This was kind of 2 x 2 channels and the 4 channel version not directly included with the ADC. 2 ADCs does however add to the costs - may still come in the future.