soundstage question

A thick blanket may have too much absorbion...too heavy and too wide. But the screen should be covered.
Panels are best for focusing the image and enhancing the soundstage; some reflective surface is needed.

A wall shelf should be fine as long as absorbion and/or diffusion is used.

4 ft. sounds like a good distance from the wall. Keeping distance from the wall helps 3D imaging, but decreases bass extension.
  If the speakers are close to the side walls, absorbion or diffusion may be needed. It’s also possible that bass traps may be needed in corners if bass does not sound tight. All these treatments (or lack of) will affect the size and shape of the soundstage.

One of my best upgrades was to use absorbion on the wall behind me. I hung a large tapestry and it helped define the soundstage and absorbed standing waves resulting in smoother highs.