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!
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!