Difference between du and Finder file sizes
Difference is in units they use.

du uses BLOCKSIZE=4096 bytes by default (rounded  if Human Readable format option -h i.e. df -h)
Finder uses BLOCKSIZE=1000 by default

$du -h /usr/lib/libblas.dylib 
4.0K    /usr/lib/libblas.dylib


and same in Finder.