Somehow, string quartets are one musical form I've never much been able to come to terms with. It has been said that composers use them to try out their various ideas on how to put things down on paper. And they often seem to meander all over the place.
True, virtually all of my musical training has been on keyboard instruments, and I admire anyone who can learn to produce music out of four strings. So I'm not knocking strings, but rather the quartet format. Yes, I know, they are a favorite chamber format.
As for getting music out of four strings, it's basically the old principle that always applies in music -- the longer the space vibrating to produce the tone, the lower the tone. Thus, 32-foot to "a fraction of an inch" on pipe organs. Maybe the trombone is the most obvious example, since you can see what makes tones lower or higher.