Will the ADA4302-4 operate at all down to 1MHz? Is there any indication of which specs will suffer most operating below 54MHz?
The first thing I should tell you is that the ADA4302-4 is on last time buy so it probably shouldn't be used for new designs unless the plan is to purchase enough parts to last the life of the product.
We do have another 1:4 splitter (ADA4304-4) but it's single-ended output, not differential. Both of these parts were only characterized down to 54MHz, however someone did take a quick look at the ADA4304-4 some time ago to see how it might do at lower frequencies. No such work was done for the ADA4302-4.
By increasing the value of the input/output coupling caps and the supply choke lower frequency operation was achieved for the ADA4304-4 but there is some low freq peaking and some input impedance mismatch. Other parameters may also be affected, it wasn't studied in depth.
This work does suggest that 1MHz operation might be possible but choosing the correct cap and inductor values could take several iterations until the best combination was reached. Even then there would likely be some trade-off between lower end BW, gain flatness, input impedance, etc.
Because operation at 1MHz is outside of the data sheet characterization conditions we couldn't guarantee any performance.