8 ohm speaker hooked up to 4 ohm post ?

Hotsauce's comments are correct if the volume control setting is identical for the two different configurations. However, it will not be, because the listener presumably will adjust it to reflect the lower output voltage on the 4 ohm taps and the higher output voltage on the 8 ohm taps.

Also, keep in mind that speakers are probably damaged more often by underpowered amplifiers than by overpowered amplifiers. The clipped waveform that can result from an underpowered amplifier being driven hard contains high frequency spectral components not present in the original music waveform, which the speaker's crossover will route to the tweeter, not uncommonly resulting in damage to it. The chances of something like that happening would be increased by connecting an 8 ohm speaker to the 4 ohm tap, because the reduction in volume may tempt the listener to increase the volume control setting such that musical peaks exceed the voltage swing that the 4 ohm tap is capable of.

That said, as long as you exercise reasonable caution with the volume setting you are very unlikely to damage anything either way. But given the impedance curve of the particular speaker, which goes below 8 ohms only briefly, and stays well above it at many frequencies, I would expect the 8 ohm tap to provide best results.

Regards,
-- Al

Adding to my previous post, if you connect a true 8 ohm or so speaker to the 4 ohm taps, you will be increasing the impedance seen in the plate circuits of the output tubes (on the primary side of the output transformer). That will not harm anything, but will tend to move the operating point of the tubes away from what the designer intended.

Most "8 ohm speakers" have actual impedances which are lower at most frequencies than yours, and also dip well below 8 ohms at many frequencies. Given that your speakers are atypical in that respect, I would not generalize too readily from the positive results others have sometimes reported when they connected to the 4 ohm taps.

Regards,
-- Al