Hi
At the moment I'm working on a string quartet myself, and I'm finding it very difficult, so I want to congratulate you on writing such a pleasant, well-constructed piece.
Unfortunately I'm going to have to agree with some of what's been said above and say that I feel that this piece needs much more harmonic variation. This is something I have a lot of trouble with myself, so I can understand what you've done, but in simple terms, this piece just needs more notes not in the key signature. With out them, sections like bars 73-97 feel like they are going nowhere, where they have the potential to be very gripping, and while the opening is very nice, the highlight is the idea in bar 4-5 as while the harmonic variation sticks out, its presence adds to the piece.
I hate to add a bit of negativity, because I overall enjoyed this piece, but I feel you need to do a bit of work to stop the listener falling into boredom.
