As long as your Smartlink has not been "Published", the Smartlink Page will display a 404 Error.
Be sure to first publish your Smartlink by going to the Smartlink on Soundiiz and selecting the "Publish" option. Once that's done, you could have to wait a few minutes before the Smartlink Page goes "Live" and no longer produces the 404 Error.
The same could have happened if you had already opened the smart link and then performed an update. You will need to wait a few minutes before your updates appear live.