OK! thanks for your reply,
This is unfortunately the problem of our entire country.

just as an answer to you, "when rar is used?" you can see a very wide usage of rar in rapidshare. ( full of splitted rar archives, usually for sharing big files )

I just wanted to say:
when better solutions are available than putting a big 4G DVD on a site, why not use them?
Yes, may be this is my local issue, but I'm completely sure that even those persons with high bandwidth internet will be able to download the splitted archive more easily.

any how,
good luck.