Mercedes is a major car manufacturer due to their corporate status.
F1 sporting regulations wrote:13.3 .... A major car manufacturer may not directly or indirectly supply engines for more than three teams of two cars each without the consent of the FIA. For the purposes of this Article 13.3, a major car manufacturer is a company whose shares are quoted on a recognised stock exchange or the subsidiary of such a company.
Under §13.3 of the F1 sporting regulations Mercedes cannot supply a fourth team. They already supply their works team, McLaren and Force India. Under the prevalent conditions (Cosworth available) the FiA is very unlikely to agree to a request of Mercedes to supply four teams. On top of that there would probably be a big stink by the horse whisperer in the F1 commission.
So Helmut Marko seems to be a bit out of tune with the regulations and the F1 commission policies.
It will be interesting to see how the over all rating of F1 engines will pan out in 2010. I have a funny feeling that Renault engines may not be in big demand compared to Cosworth if Cosworth continues on good reliability.