Do stints really need to stay in for 4 wks after septorhinoplasty, turbinate reduction and septal perforation repair.

Keeping the stents in for 4 weeks seems a little excessive. Typically, stents are kept in place for 5-7 days (varies between surgeons for sure) and are helpful for reducing the risk of collecting blood in the nasal septum (hematoma).
If kept in place too long, one would be at a higher risk for infection from the foreign body as well as causing a septal perforation (hole in the septum where the stents are sutured in place).
It would be important to discuss further with your surgeon to make sure that there is no miscommunication as well.