Since you posted your question in "Localhost Installs", we'll be making the assumption that you have full access to the server-side php.ini file, and "contact your host" does not apply.
"upload_max_filesize = 128M and post_max_size = 8M"
Let us just take a minute to review what we have talked about. You have said that your WXR file is only 4MB. I have suggested that you change both of the above settings to 10M each. If that is the problem, that should be sufficient to make it work.
While you have chosen to change the settings to: upload_max_filesize = 128M and post_max_size = 8M, that will probably also work - if it is the problem. One thing you may need to do after making those changes, is restart apache - or whatever web-server you use - to make sure the changes take effect.
Now, what i will do? Please tell me solution?
Restart the web server and try to import the file again. If that still won't work, you may want to read this information and see if any of the suggestions you find there might help: How do I Import a WordPress WXR file when it says it is too large to import?