I believe you're showing the measurements for the 3 not the 4. Regardless, did you read J.A.s comments?
"....The tweeter is almost 5dB too low in level in the octave above the crossover frequency—this will be compensated for when the speaker is driven by a tube amplifier having a high source impedance—but returns to full level above 6kHz and to an increased level above 15kHz. How this affects sound quality will also depend on the speaker's radiation pattern. The Sonist's horizontal dispersion, normalized to the quasi-anechoic response on its tweeter axis, is shown in fig.5. The off-axis behavior is surprisingly even, though a notch develops to the speaker's sides at the frequency of the on-axis peak at the top of the woofer's passband. There are also some off-axis peaks at the frequencies where the tweeter has some on-axis notches. All of this means that the room's reverberant field is not going to be as uneven or colored as you might expect from the on-axis response....."
Anyone using transformer coupled SET tube amplifiers probably isn't too concerned with accuracy of reproduction anyway. These particular speakers were designed to be used with amplifiers that have higher output impedance.