Something

#!/bin/sh
url=http://ftp.gnu.org/pub/gnu/aspell-dict-it/aspell6-it-2.2_20050523-0.tar.bz2
#get length
lengthstring=`curl $url -I | grep Content-Length:`
length=`echo "$lengthstring" | cut -d" " -f2 | sed 's/\r$//'`
echo $length
#parse length
truncateby=`expr $length % 4`
truncate=`expr $length - $truncateby`
stdlength=`expr $truncate / 4`
end1=$stdlength
start2=`expr $stdlength + 1`
end2=`expr $stdlength \* 2`
start3=`expr $end2 + 1`
end3=`expr $stdlength \* 3`
start4=`expr $end3 + 1`
#download
#curl  -r 0-$end1 $url -o filename.part1 -w finished
#curl  -r $start2-$end2 $url -o filename.part2 -w finished
#curl  -r $start3-$end3 $url -o filename.part3 -w finished
#curl  -r $start4- $url -o filename.part4 -w finished
#reasemble file
#touch file.tar.bz
#cat filename.part1 > file.tar.bz
#cat filename.part2 >> file.tar.bz
#cat filename.part3 >> file.tar.bz
#cat filename.part4 >> file.tar.bz