Firstly, why do people choose SETs, it's because they're a distortion circuit. They have a characteristic sound, different from low distortion circuits, whether SS or tube. Why I don't understand is why anyone would buy a low distortion tube amplifier, given that then it would sound like a SS amp, but at 10x the cost. HiFi for some has nothing to do with High Fidelity, it's High Pleasure, and if they like the sound of distortion, so be it, their money, their (daft) choice.
As to 4 ohms or any other impedance, with tube circuits is doesn't matter, as the output transformer is tapped for 4, 8 or even sometime 16 ohms. SS amps will drive more power into 4 ohms than 8 ohms, (theoretically twice the power) but tube amplifiers will have the same power into 4 or 8 ohms provided of course that the correct tap is used....if it isn't the amp will distort even more.
Why would anyone buy a Rolex watch when a Casio keeps better time....clearly, a Rolex isn't bought for its timekeeping qualities any more than a SET amplifier is bought for its fidelity.
S.