Voltage drop on relay coil while driving relay using Darlington Arrays
The relay is guaranteed to operate at 75% of nominal voltage, which is 3.75V. 4V is greater than 3.75V, so the relay will operate under the specified conditions. However.. 

The reference conditions (which you did not copy) are that the ambient temperature is 20 degrees C. 

Even if your ambient is a comfortable 20 degrees C and no more, the relay coil will heat significantly after it's been operating for a while. Copper has a tempco of about +0.4%/K, and relays are actually current-operated devices so it won't take much temperature rise before the relay is no longer guaranteed to operate at all. 

The lifetime is specified with nominal voltage (and, incidentally, without the flyback diode). Both differences will tend to make the relay operate more sluggishly and may reduce the contact life. 

The same is true of a higher voltage relay, but the current is lower so the Darlington will drop a bit less voltage typically, and 1V is much less percentage of 12V than of 5V.

There is a bit of a compensating effect for actual ambient (as opposed to self-heating) changes in that the Darlington will tend to drop a bit less voltage at higher temperatures. 

Especially for the lower voltage relay, it's better to use a small SOT-23 MOSFET to do the switching and a discrete flyback diode or a diode in series with a Zener diode to speed up the contact opening. For example, an AO3400A. With your 3.3V drive voltage, that particular MOSFET has less than 0.05 ohm resistance compared to the 55 ohm resistance of the coil so it will have negligible voltage drop.