If you have not already liked a File posting, you can do so by sending a request to the file entry url. Here's one easy way to do it with an asynchronous HTTP (Ajax) request using jQuery.
There is no a link to IBM Connections 4 documentation.
Atom API Request Details
|Response||Status Code: 201 Created|
Example using jQuery
Note: console.log can throw errors in IE, they are there for illustrative purposes only and should be changed as required.