[morzh]

Rod

Yep, and 6 ohm still make it 4&6

It is all pretty approximate.
Not only that, but if one wants to be real anal about these things, the transformer itself behaves in a pretty interesting fashions depending on the frequency range.


At low frequencies the impedance is pretty much that of the reflected resistive impedance of the speaker and the losses of the output winding, paralleled by the equivallent core losses and the primary inductance impedance, and then added to (serialized with) the losses (DCR) of the primary.


At midrange, the impedance is pretty much the same without the primary inductance factor.

And in highs, it is parallel of parasitic capacitance of the windings and the core losses, serialized with full stray inductance (prim and sec), and reflected resistive losses and the resistive load.