How do I get HTTP headers in 4.0.4?

I need to grab the authentication headers from the raw HTTP request. 
I tried $this->request->header() but I get a 500 error:
Call to undefined method CodeIgniter\\HTTP\\IncomingRequest::header()

$this->request->getHeader('Content-Type') returns empty (via Postman)

Also, where is the 4.0.4 User Guide?