Parece ser que mantienen las impedancias, o que al menos no descienden de esos 4 Ohmios. Fonestar no de ninguna información al respecto, pero no es la única marca que tiene este tipo de dispositivos. Te dejo un link de la marca Monacor, que en las instrucciones indica lo siguiente:

1 Applications
The speaker switch box allows connection of up to
eight speaker pairs (see fig.) which can be switched on
individually or in any desired combination. It is possible
to use speakers with an impedance of 4 – 8
Ω. The
switch box is able to handle a music power of up to

300WMAX.

Caution!
Never connect 4Ω speakers, not even via
the switch box, to an amplifier with an output impedance

of 8Ω. The amplifier may be damaged.

Due to the internal protective resistors it is always
ensured that the total impedance does not fall below
4Ω

Y ahora el link:

http://www.monacor-spain.net/modules...ticulo=SPS-80S