Dumb lo-fi question.

I believe you are assuming correctly. Receivers of that era, and probably now also, would parallel two sets of speaker terminals.  Generally, most speakers, I say most, not all, run a nominal impedance of 6 to 8 ohms. If you run four speakers that are 8 ohms nominal (average), your total impedance would be approx. 4 ohms. The receiver might put out more than it's rated power of 25 watts per channel, which should be ok for that receiver. If you have the owner's manual they might caution you about running four speakers with impedances lower than 8 ohms. To low of a speaker impedance load, and you will wind up frying that receiver, especially if you crank up the volume.

I have a Harmon Kardon receiver with the same capability to run  2 sets of speakers, and it pretty much told me what I have told you.
Good luck!

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.

I ran across a review of the old Toshiba (SA-2500) that I have. It is a contemporary review and says a lot of good things about this receiver. 

http://www.hifi-classic.net/review/toshiba-sa-2500-322.html

I guess I wonder why there are two sets of outputs on a unit of that vintage? Is that a holdover from the old idea of "quadraphonic" sound?