Dumb lo-fi question.

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.

organicsound, you mention bi-wiring one set of speakers by using A+B.

I just put in an offer on a set of low end Polk Audio towers with have double connectors on the back and claim to have a separate crossover for the two woofers (that they call subwoofers) while there is another crossover for the two mids and the tweeter. They are 8 ohm speakers.

On such low end speakers and with my old receiver is there any chance that bi-wiring this way would provide any benefit?

I'm assuming that this would not put any additional strain on the receiver?