What does the Sun look like from the heliopause? I calculated the angular diameter of the Sun at the heliopause to be 0.004°.  If that is correct, would the Sun appear as a disk, or like the pinpoint of light from a star?  

How bright would the light from the Sun appear?  I understand at Pluto the Sun appears as bright as the full moon at night on Earth.  Assuming the heliopause is 3x distant from the Sun as Pluto, does that mean there would be 1/9th as bright at the heliopause.  What would that brightness translate to an everyday measure?
0.004° converts to about 14.4 arc-seconds.
That's within the range typical for Mars; 5-25 arc-seconds. 
You'll see the sun as a point with the naked eye.
Heliopause is about 14 billion miles out. Absolute magnitude of sun is 4.83.
14 billion miles = 0.0007297pc.
Apparent magnitude will be -15.8543. Magnitude of the full moon, from earth, is about −12.74 
So you'll see a point source of light, brighter than the full moon.