electronic switch, understanding on-resistance measurements
Vd seems to indicate the voltage on the D inputs (D1 through D4).  Presumably it means the voltage on the Drain of the switch devices.


  I'm asking since, as far as I know, on those switches the
  on-resistance should raise as the voltage to be switched get close to
  the rail voltage.


You are perfectly correct but the switch is actually made of two devices in parallel - one N-channel FET and one P-channel FET.  As the input voltage (Vd) approaches the positive rail the N-channel device does get higher in resistance but the P-channel device gets more bias since its gate is driven by the negative supply rail.  The combined resistance tends to be the lowest when the input voltage is mid-way between the rails when both devices have significant bias.

Be careful about reading the data sheet test conditions.  They state the supply voltage, input voltage range and temperature. Maximum values should be guaranteed over those ranges.

I can't see what particular conditions the 4 ohm flatness or 35 ohm maximum apples to  - it seems to be 45 ohms with 15v supply (-10%) over temperature.