It depends on the tube amp. The pre-UFO Zens love low impedance as the lower the impedance goes, the more power it puts out. The UFO Zens and the Kitoki both have 4 and 8 ohm taps on their OPTs so 4 ohms shouldn't present any problems. My guess is the other Decware amps with UFO OPTs should also work well with 4 ohms. I can't speak for other tube amps though as what you say may be the case in some instances.
It sounds like the pre-UFO's had a high impedance output transformer, and as the speaker impedance fell the reflected impedance to the output tube approached a more ideal operating point for the EL84 (5K or so). What we would expect to see is a low power into higher impedance speakers and higher power as the reflected impedance fell into a range that extracted maximum power from the tube (probably somewhere around 3K). I looked around and Steve's own words confirmed this:
"Our custom hand wound air-gapped grain oriented silicon steel output transformers have a primary impedance of 9800 ohms and the EL84 / SV83 output tubes would rather see 1/2 to 1/3 of that. This combined with a single 6 ohm tap gives ideal clipping characteristics. on the average speaker load. Lets face it, 8 ohm speakers are often closer to 6 ohm and with a six ohm tap the Zen can drive either 8 or 4 ohm speakers. The clipping characteristics. of this design are so graceful that you can't hear when the amp is soft clipping. It takes a hard flat line clip before you can actually hear it."
He goes on to say:
"In the tests above, using the same output tube, the Zen Triode amp performed with higher RMS output into low impedance loads. For example, a 16 ohm load yielded just under or at 1 watt RMS before measurable (not audible) distortion. (The waveform shown in the picture) An 8 ohm load yielded around 1.5 to 1.9 watts RMS. A 4 ohm load gave us around 2.38 watts RMS and a 2 ohm load gave us around 3.21 watts RMS."
Generally speaking, higher-impedance speakers will yield lower power and lower distortion in this application. The textbook operating point is using speakers at 6 ohms, with higher power output at lower impedances, albeit with higher (second order) distortion.
All things being equal, a higher impedance speaker and output transformer is easier/better for sound. In a single-ended amplifier, you are pushing it to get the output impedance down to 1 ohm with 8 ohm speakers. 16 ohm speakers make it easier to get better bass response and require less current from the output transformer.
EDIT: I will add that it would appear that the higher sensitivities of the two-driver Omega's are in part due to the using a stated voltage of 2.83 volts, equivalent to 1 watt into 8ohms. Halving the speaker impedance doubles the voltage sensitivity of the speaker but does not actually increase its acoustical efficiency. You're still only getting another 3dB per watt, not 6dB. If you're able to swing the additional voltage on the lower tap of your tube amp (if it has one), you will actually get more power, but it may be at the expense of reduced bass performance or greater distortion.
In the case of a HO Omega connected to a Decware Super Zen, you may have an ideal amp-speaker system: putting drivers in parallel gives you an additional 3dB of acoustical efficiency, reducing the power demands on the amplifier in most scenarios. Additionally, the lower impedance puts the amplifier into its sweet spot of maximum power output. This comes at the expense of higher distortion at full power (which is largely academic, as the second-order distortion products are generally benign)
however with the increased efficiency you will be operating at lower output power most of the time, and this higher distortion will be even less common.
In this scenario it would appear we can have our cake and eat it too.