Due to the large number of files and their sizes, the release of SF4 for
the United States has been broken down into smaller groups by 
iterations.  To make downloading easier, each tarfile is under 2 gigabytes
in size.  

For example,

US_001.tar         contains only the files for iteration 001.
US_038thru207.tar  contains iterations 038 through 207.

Each tarfile contains a copy of the usgeo_uf4.zip file also.