I am pretty sure it would still be 4V to both. A splitter should just split the signal, which would mean the two amps are in parallel to the empeg. Therefore they would both still get the full voltage of 4V, just less current.

In reality you'll get about 3.96 (bogus number taken out of thin air) instead of 4V.
The exact difference depends on the output impedance of the empeg and the input impedance of the amps.

In the case of one amp you have the (ideal) voltage source of 4V voltage dividing over the empeg output impedance and the input impedance of the one amp.

Adding a Y-splitter and a second amp will change this to a voltage division over the empeg output impedance and the parallell connection of the two amps input impedances. The equivalent impedance of the parallell connection of the two amps will be lower than the input impedance of the one amp, thus causing comparatively less of the voltage to fall over it.
Or looking at it another way; because the current draw increases due to the lower impedance load, the voltage drop over the empegs output impedance increases...

Just increase the gain slightly, if you detect a difference at all.

/Michael