I have a strange problem with displaying assets like documents in a new tab in SS2.4.7. I had this working and then the client wanted a different domain so I had to switch everything to a new location on the server so I could migrate. Now where I have instances of documents that need linking from text, they won't open when you click on them.
This could be a million different things. And probably not Silverstripe related. The fact that you are getting a 404 suggests the files simply do not exist where the links say they do since the move. If you had a 40x (other than 404) or 50x error, you could at least say it was a config issue of some sort.
This could either mean the links are incorrect, or the files simply aren't in the expected location, if at all.
Do you have a link to a live site? Can you view any other type of file inside /assets?