The eight π loudspeaker is one of the most tube-friendly speakers in the line. You're right that impedance falls to a low of just above 4 ohms at DC and very low frequencies, but it quickly rises above 8 ohms.

Tube amps are actually more sensitive to impedance peaks than dips in most cases. I'd suggest these speakers be put on 8 ohm taps, but there may be some amps that work better on a 4 ohm tap.

Transistor amps sometimes become distorted with very low load impedance, and some can even be damaged if the load drops too low. But 4 ohms is generally acceptable.

The eight π loudspeaker is over 8 ohms from 100Hz up, over 4 ohms minimum and less than 30 ohms maximum, so it is not a difficult load for most any amplifier to drive.