it's pretty similar to the google searches.
go to google.com
type in : site:https://www.4shared.com/folder/ (insert something after the last "/" like a word or letter.
It does seem like most of the content on 4shared is foreign, so perhaps words in Portuguese, Spanish or Hindi may generate further results.
When you run out of search terms you can broaden the scope by just putting in 1 or 2 letters or removing "/folder"