>1) Why they don't accept site of google rank "zero"?
because some people confuse pagerank with value and do not know how to tell if a site is banned or new.
>2) How can I improve my site ranking to above 4 and How can I know what is current my site ranking
get a bunch of inbound links. start w directory registration.
>3) What should I do next?
buy some inbound links from directories and other sites
what directories were giving you nasty responses based on PageRank?