Shallow DOF not so shallow?

[knightrider]

I follow Sam Javanrouh's daily photoblog because he takes some really nice photos, and because he always includes the camera settings used, which is really useful to learn from. Today's image confused me, however.
I've got the impression that f/4 is a pretty fast aperture, and should give a fairly shallow depth of field. But in this photo, shot at f/4, the depth of field is seemingly infinite. Both the rigging on the yacht in the foreground and the buildings in the background are in focus.

See Photo Here

Have I misunderstood the use of aperture in creating a depth of field, or is there something else in the way this photo has been taken that means it is so?

Thanks