These are just things I typed down listening to it straight through.
1st Movement: A very nice a mellow mood that really works. Unless intended, there are a few dissonant spots, but I personally like that!
2nd Movement: The beginning buildup was nice; I think developing that a bit more could be beneficial (pizzicato might work better, but it sounds good that way too). The buildup at measure 78 was very nice!
3rd Movement: Ok, so this pizzicato is amazing starting off. The lightness of this movement is very apparent; really liking the conversation between the violin and violincello.
4th Movement: Some more variety in the violin II and viola part would be nice to not keep it so monotonous. Changing up the chord progression to make the movement go somewhere could be cool too!
Overall Thoughts: Although I have very little experience with string instruments (I'm a percussionist), this piece I think is a really good foundation for something bigger. You have a good amount of the right elements there, I just think some more work into the melody, and complexity of the parts would make this piece even better. I plan to write a string quartet soon with some piano, and you have definitely gave me more motivation to do so! Keep it up! 😄