Here is an example: the given number found in a literature is 4021865.

You may use Google Patent, Espacenet, or Patentscope, simply type in the number in the search box to locate the patent document. In this case, the actual patent number shown in the patent document via Google Patent is "JP4021865B2". Notice the last two characters "B2". That is called "Kind Code". It indicates the status of the patent. "B" = issued patent. (See more info on Kind Code definition.)