I love my NST, but it sounds like you don't really need it, and most of your stuff is probably in MOBI format? It might be easier to stick with the three Kindles you have. Why did you get another e-reader?
Both my NST and my K4 seem to be high quality. The NST seems a bit more substantial, but OTOH the K4 is much lighter and there are serious advantages to that. I think it's a wash as far as that goes.