"Four Way isn't the term, and means something else in audio-ligua. what you had was a Quadraphonic, or four SPEAKER system. Today, the base system is, in your terms, "Five Way", plus a special effects tap for a woofer/amplifier-in-a-box add on. The fifth channel feeds a center speaker that usually hands out voice tracks, so people can put a speaker underneath the TV to enhance the illusion that actors are speaking out of the TV. The center speaker is most helpful when the left and right are too far from each other to center the dialog properly, given the room space and acoustics, Anyway, its called Surround sound.
Very old PCs may only have a four speaker or quadraphonic ability, usually synthesized, not really handling five separately recorded music tracks, one for each speaker. All recordings are mixed so that hopefully, a stereo effect - the illusion that your two ears "see" instruments and events positioned in the room space, not blaring from the speakers, is the point. This means the instruments played CANNOT be confined to any one speaker, but MUST be distributed so that the apparent source of the instruments seems to your ears to be at one place or another IN THE ROOM. This illusion can be very strong, if the speakers are very good, and placed properly considering the room. An instrument blaring out only from one speaker would tell your ears that the sound was coming right from that speaker which is largely not desirable. The rear speakers generally add only an additional sense of space, as in, behind me. The left right mixing illusion is done in the recording studio, and expects the front to have a single left and one right speaker to reproduce the illusion correctly, and the engineer works in the center channel so as to hopefully not disturb the magic to much. The rear image similarly. Since desktop speakers are rarely more than four or so feet from one another, it is usually not necessary to have a center "fill in", or fifth channel.
Bad speakers or poor placement of them kills the music playing in space - not from the boxes, illusion. So you don't want to mess with it by using "more" speakers. It just screws up the sound engineers hard work. How clear and precisely placed the individual instrument will seem depends on the quality of the speakers, not their number. Ten skunks do not smell like a rose just ten skunks.
Six , seven, Et all type surround systems can divide the sound up further, but in the main, are useful for large rooms, increasing the number of side-wall speakers, which puts more distant listeners closer to the ambience, or "rear" sounds. They can also include more direction clues to fool your ears, and aid listeners in big spaces like in movie theaters, that usually have this problem due to their size. .