Firstly, most string quartets are comprised of two violins, a viola, and a 'cello. Depending on the piece you select, all of those parts may be pretty independent of each other. Usually if you have a double bass, you're talking about a chamber orchestra, where you'd most likely have five independent parts.
The easiest way to do would be (1) analyze the piece to get the chord progression, (2) isolate the melody as much as possible, (3) re-arrange the piece to have the melody on top and a semblence of the progressions underneath. It can be done, and as long as you highlight the melody, people will recognize it for what it is.
Good luck, and post it up when you finish it!