I have already combined four CSS code into one file and have it uploaded into my directory. I have added a link in the header.php. All of the four individual CSS files are in directory. Now do I have to remove or delete the four individual CSS files? The four are: style.css, custom.css, plugin related ccc and minified index css.
Anytime you are not sure if you will need something you should just save them in an archive outside of the webspace. then test an make sure everything works without them. As ;ong as you keep the copies in an archive you have the kind of version control that allows you to recover if a problem shows up at a later date.
Since you combined the 4 into that single file (which is always a good idea to reduce the number of HTTP requests) you can now delete the 4 individual files since they are no longer being used.
Yes you can remove the other 4 styles and use only one combined CSS. Even if you have 4 css all the css will be requested only once during the first time a page is loaded and will be saved in the local folder for style reference. It will not be requested again and again. That is the use of CSS.