The last paragraph of what millercarbon said is what matters. The amp will work harder driving 4 speakers compared to 2 so if you try to go as loud with 4 versus two, you will probably find the sound quality deteriorates and the amp will start to clip sooner, so be more careful in that over-driving any amp into clipping not only affects sound quality but can also damage speakers.
Dumb lo-fi question.
I have an old (1980) Toshiba receiver that I use down at my cabin. Have been pondering a slow....very slow...upgrade path of that lo-fi system and I have a question. The receiver is rated at 25 wpc. It has two sets of left and right speaker outputs. In other words it will drive 4 speakers. Does that mean it will send 25 watts to each of the four speakers or 12.5 watts to each of the four speakers?