... with the Sensor+ technology (which must be done on chip), 4 pixels are combined into one super-pixel which can be read with only one read noise, resulting in an improvement in the S:N of 4:1.
That sounds right as far as the dark noise arising from transportation, amplification, and A/D conversion, but not photosite noise (dark current?). But these are probably the dominant sources of dark noise, so gaining about two stops of DR at equal ISO speed seems reasonable.
I would think that dynamic range would be improved, but Phase One claims a DR of 12.5 stops for both methodologies, but the ISO is quadrupled in the Sensor+ mode.
Roughly, quadrupling ISO removes two stops of DR because the signal strength is reduced by 1/4 while the dark noise level stays the same, so it makes sense that the effects on DR of 4-1 binning and quadrupling of ISO balance out. There should be a DR gain at equal ISO speed (400 or higher) with and without binning.
Why does Sensor+ not allow use of the lower ISO speeds? It seems that either the signal path after binning is limited in charge/current capacity to what a full well gives without binning, or the amplifier (charge to voltage conversion, ISO gain etc.) is limited to this DR. The blame cannot come after the amplification and charge to voltge conversion, because the amplification could be adjusted to keep the output within what subsequent components can handle, and it is hard to see 14-bit or 16-bit ADCs in backs this expensive imposing this 12.5 stop DR limit.
So once again, I see evidence suggesting that a major DR limitation is that the DR of the amplifiers is less than that of both the photosites or the ADC.