> The MD5 generates a long string. Did got only the 
> first 4 bytes?
Yes. The md5 is a really powerful algorythm, and 
therefore the "entropy" is mixed over all bits - that 
is, you have (I simplify) for each bit the same 
interference probability with the source data. (I 
don't know if my english is really good, hum)
Therefore, you can "cut" a md5, and you'll get another 
hash function, which has the same qualities (but with 
a lower space cardinal)
You can also get the modulus of a 128-bit md5 result, 
for hashing purpose (using a prime number, for example)

HTTrack generally gets the 16 first bits, and convert 
them to name the files containing a query string