Dumb lo-fi question.

glupson, good catch. I never noticed that. My only real interest in this was possibly putting a couple of small towers in the middle of the room and my existing Bose 2.2s on the flanks. Totally unnecessary for a room that small but just a thought. Given the limitations of the receiver I have no intention of doing that now.

As a side note...I mentioned this in another thread...a while back several of the pots would cause static and distortion when turned, particularly the volume and I was about to toss it when someone on another web site mentioned a spray for pots and terminals and such. I bought some and opened up the receiver and sprayed the various controls and now everything works perfectly. The only thing that does not function is the tiny light on the channel slider so to change radio stations you have to look very closely or get a flashlight out. The inside is pristine. A miracle since this thing went to college with me in the balmy south in rooms with no A/C for 4 years.

First encounter with stereo equipment was 1968, Yokuska Japan, building A33.  Every Japanese Stereo manufacturer was on display, clean, well lit.  They all had lots of knobs and switches, dazzling backlit displays.  The display stands  were large round circles that drew me into their glowing wonders.  Sleek black/chrome blue light wonders.  Some had terminals and switches for up to 3 sets of speakers.  Abundant brochures from each company, touting the virtues and attributes of each unit.The American manufacturers were haphazardly placed on a table, unlit.  There was a total of 3 pieces on the table, an Acoustic Research receiver and integrated unit.  One piece by Lafayette or Harmon Kardon.  All were transistor, there were no brochures.I chose a Pioneer SX525 over a JVC Nivico.  There was zero McIntosh on display.  Years later the disparity of the displays has me still wondering about the difference in the displays.  5 years later I replaced the Pioneer with a Acoustic Research integrated unit, a big upgrade at the time. 
N80,  integrated puts it all under one hood, separates give you a lot of flexibility.  No mater what choice you make, enjoy the music, that is why we are here.

To answer the op’s question...... 25wpc divided by 4 speakers is 12.5 wpc in 4 channel mode. 
Ive had plenty of vintage equipment and the power drops dramatically with 4 speakers in a plus b mode. Also, watch the speakers rated impedance. Running 2 or more 4 ohm speakers in ab mode can fry the amp..

The amplifier will still be rated at 25 watts per channel no matter if one or both pairs of speakers are connected.

The sound output level will correspond to 25 watts of power being consumed. If one pair is connected and playing at full power, the SPL is determined by the 25 watts. If two pairs are connected, the power consumed by each pair is 12.5 watts (assuming identical speakers) and the resultant SPL drop is 3dB per pair. But when you add the SPL of each of the speaker pairs, that raises the SPL by 3 db. Conservation of energy.

I have some older amps with the A, B or A+B switching.  The best way to use the speaker options, is to use a set of speakers in two seperate spaces (rooms/areas). They would normally be switched either A or B, depending on which room/area you were in.  The other option of A+B can be used for bi-wiring one set of speakers if that is desired.