@lloydc Efficiency is 1 Watt/1 meter. With this spec, the speaker is fed 1 Watt for the measurement.
With Sensitivity, the speaker is fed 2.83 Volts instead. 2.83 Volts into 8 Ohms is 1 Watt. Into 4 Ohms its 2 Watts. So subtract 3dB from the efficiency of the speaker you have now, and you'll see there's only a 1 dB difference.
For tube amplifiers the Efficiency spec is more useful, since tube amplifier do not double power as the impedance is cut in half.
But there is another issue here, which is the how the amplifier interacts with the impedance! Your amp uses an output transformer, and output transformers are always less efficient driving a lower impedance load!
They also can lose as much as an octave of bass response between the 8 Ohm tap and the 4 Ohm tap. In addition, the amplifier will have lower distortion driving 8 Ohms.
Since the difference between the two speakers is really only 1dB (if you do the math), you won’t have any trouble in that department. So the impedance is the far larger variable. I say go for it since the amplifier will be working better (if you measure the temperature of the output transformers before and after, assuming the same warmup time, you’ll find they are running cooler; that extra heat is caused by greater signal losses in the transformer).
In short I don’t see a downside with this move.
