This lens is actually at it's best with F4 and I'm surprised the DOF wasn't more shallow.
Do you see anything in the EXIF info that I could have done differently?

The problem is Loueen is that at 28mm focal length and F4 depth of field is quite deep as the wider the angle of the lens the more DOF there is even at larger apertures. In this case a longer focal length lens with more distance between you and the subject would have thrown the background out of focus.