> I would like to know how is created the 4 byte MD5 
> string. Or all MD5 strings that are used to create 
> each file name.

..using the MD5-128 hasing algorythm. You can take a 
look at this algo in md5.* files and htsmd5.* files. 
If you are interested by the principal, see the 
excellent book of Bruce Scheiner (applied crypto)