Showing "404 (Not found)" Error at Google Webmaster Tools

I’m having problem in my Google Webmaster Tools under “Crawl errors” section.

There are showing some “404 (Not found)” errors for few pages that are not existed. When browsing those links, they are going to 404.php page[I’m using Word Press].

There is also a "Not followed ‎(1)‎ " Redirect error and also that page is not existed.

In .htaccess, I’ve used

ErrorDocument 404 /index.php?error=404

But, Webmaster tools still showing those errors.

Please help me so that I can fix “404 (Not found)” in Webmaster tools.
Thank you so much.

Your server (and Google) are doing what they’re supposed to with regard to sending them the 404 code for pages not found. You should be able to get around that by using a mod_rewrite redirection after testing !-f (does NOT exist as a file) against the %{REQUEST_FILENAME} then redirect to index.php?error=404 but use an R=303 flag (temporary redirect, if any) with the Last flag.