Dumb lo-fi question.

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.

If you don’t want to take a chance on cutting your impedance in half and possibly fry the amp or destroy the speakers, get an external speaker selector box with matching impedance so the amp sees an 8 ohm load even with 6 speakers playing. PM me and I can tell you what units I have used for whole house (master bedroom and bathroom, library room, outside, living room) audio using Yamaha or onkyo receivers or go search for matching impedance selector boxes for 4 or 6 pairs of speakers.