Removing duplicate content caused by node/nid/talk pages
So /node/4 is your about page isn't it? you just added a url alias. Which means you can access the same page with /node/4 or with /about-us

To quote Clive from Non-existent URLs in Views 3 in drupal 7


  That's the expected behaviour - Drupal automatically routes you to the
  'parent' if it can't find the 'child' part of the URL


when I go to mywebsite.com/node/1/talk it displays the /node/1 content to me; even though, the url in the web browser says mywebsite.com/node/1/talk, and I haven't even installed the talk module at all.

Try going to mywebsite.com/node/4/ijustmadethisup and you'll see you get displayed /node/4

Therefore, this is just the default Drupal behaviour, nothing to do with the talk module.