Tried to upgrade from version 4 beta 2 to 4 RC2 and got the following error:
* Component Install: Another Component is already using directory: "/xxx/xxxx/xxx/xxx/components/com_comment"
* Component Install: Another Component is already using directory: "/xxx/xxx/xxx/xxx/administrator/components/com_comment"
You are supposed to Uninstall JoomlaComment Beta 2 before installing RC1. As long as the option in the backend "Uninstall complete mode" is set to No (the default), then all settings and comments will be kept in database and only the files are deleted (so you can then install RC1).
JosComment is not made by us, and having it enabled (or even installed) conflicts with JoomlaComment.
EDIT: Apologies, I forgot that the plugin filename for JoomlaComment is indeed 'joscomment'. Yes you need the plugin, otherwise nothing will happen at all that's what makes the Comment component "attach" itself to your content items and stuff.