Use a new css file to override website's css

My website has 3 css files that are automatically included as a part of the website and I do not have access to the source i.e. index.html of the website but I do have access to the css files of my website. I am trying to use my own style to override my websites css files and create a new css file that would contain all the styling that I would like to overwrite on my website. I have tried using @import url(css4.css) and I have placed that at the top of my last css file but that wouldn't overwrite the last css file's styling. How can I achieve this? Any suggestions please?

<link rel="stylesheet" type="text/css" href="css1.css">
<link rel="stylesheet" type="text/css" href="css2.css">
<link rel="stylesheet" type="text/css" href="css3.css">

....
<link rel="stylesheet" type="text/css" href="css4.css"> // How can I achieve this just by using css?