Halving the impedance doubles the current (amps) required. Tube amps are current "challenged" compared to SS amps.Therefore the higher the speaker impedance the easier it is for a tube amp to provide enough current.The 4 ohm tap allows the amp to make the current needed. The problem is that limitations in the output transformer often mean that the transformer runs a bit hotter when running the 4 ohm tap, and this extra heat comes from the power of the output tube itself. IOW, the amp will make slightly less power due to its being less efficient. But it will make the current needed to drive 4 ohms.
IMO/IME there really is no good reason in high end audio for 4 ohm speakers regardless of your amp. They allow you 3 db more output if you have a solid state amp, but **all** amps make more distortion into 4 ohms as opposed to 8 or 16 ohms, and that distortion is audible as brightness and additional harshness, neither of which is desirable especially in high end audio.
If you really want to do your amplifier investment dollar justice, a higher impedance speaker will do that better all other things being equal. In the case of an SET, I recommend a higher efficiency speaker too; an 845 doesn't make a lot of power as amps in general go, so about 98db 1 watt/1 meter would be my minimum recommendation if you really want to hear what the amp can do.