I don't know if you would be able to download sheet music of "What a wonderful world" for string quartet for free, but I did find what appears to be reputable paid versions here and here
- "What a wonderful world" is a beautiful song which was originally offered to Tony Bennett but for reasons best known to himself (I bet he is regretting it now) he turned it down.
- It was recorded by Louis Armstrong and released as a single in 1968, making Louis Armstrong the oldest male to top the charts at 66 years old.
This song sounds beautiful when played by a string quartet. I found the video below on YouTube, though I am sure there are many others too.
I enjoyed listening to it very much, I think you will too.