echo 3 > /proc/sys/vm/drop_caches clears pagecache, dentries, and inodes.
The 200mb you have unaccounted for is probably mounted extensions. The 4096 block size save a bit of ram over the 128kb block size, but depending on how many files are in each mounted extension, there is still ram taken up when extensions are mounted and the files in each extension are read to be symlinked.