Hi Nikodimo,
you surely know about parallel connection of speakers and what happens to the resistance then!?!? The resistance is halved. So with two speakers connected in parallel with 4 ohms, 2 ohms per channel would put a lot of strain on the amplifier or destroy the power amplifier transistors. Fortunately, there are power amplifier protection circuits these days.
I also played it safe and connected a load resistance of 10 ohms per channel in front of the speakers. The small boxes are also available to buy (see link). It does not affect the sound in any way.
So I use 2 speakers (4 ohms) for the left and right channel and my pre out is connected to my sub. The whole thing is called quadrophony. (But be careful not to use four of the same speakers if your listening position is at the back wall of the room. Then the rear speakers would dominate and the front speakers would be hard to hear.
I use speakers with a stronger effect at the front than at the back.
For example.
Front 4 ohm efficiency (2.83V/1m): 88.0 dB
Rear 4 ohm efficiency (2.83V/1m): 86.0 dB
https://www.amazon.de/dp/B07S1XHF42?psc=1&ref=ppx_yo2ov_dt_b_product_details
