Sometimes you have to do this via a File Manager tool provided by your webhosting provider. In cPanel, it's called File Manager. In other tools it might be called something different. In the file manager, you can simply navigate to the file you want to alter permissions for, then click on the File Properties link/button and set permissions. The links could have varying names.
If your FTP program won't let you set CHMOD 444 and 644 doesn't work successfully, you'll have to use the File Manager approach above. If that doesn't work, then you must contact your hosting company's tech support to ask them to do it for you. Sometimes, although rare, it's necessary to set these files to 400 instead of the common 444 or 644. Any lower than 400 may cause you to lose access to the file.