The Joomla 4 compatible version of JComments from estreme can be downloaded here.  Please also see the jDownloads Documentation for more details.

JComments 4 is an extensive comment extension for Joomla 4. To use it with the jDownloads series 4 you must download this file.  Afterwards copy the unzipped file, com_jdownloads.plugin.php,  to: components/com_jcomments/plugins (and overwrite the old version).   jDownloads version 4.0.17 beta (or newer) is required.

You have the correct version if the archive file name is 'com_jdownloads.plugin_4.1.zip'. 

Make sure that the option "Activate support for all Joomla content plugins?" is activated in the jDownloads configuration Frontend tab
Note: Comments are only shown in the Download Details page. But the amount of comments can be viewed in the Categories, All Downloads and My Downloads page. Do not use Komento and JComments at the same time!