Why does gravitational lensing produce four copies of the star and not a whole circle of copies? I would have expected the light to bend evenly around the intermediate star, giving a halo-like effect.
Not all lenses are good lenses, especially not the ones nature makes by accident. It also matters where the background source is relative the centre to the crappy lens.
A favourite human made example seems to be the base of a wine glass, that manages similar levels of awfulness, see third picture here for how the same basic shape can make different images of the same source: http://inspirehep.net/record/850223/plots#
I was also interested. It seems to be a combination of the intermediate galaxy being elliptical, and the star behind it being off-center from our point of view:
"While gravitationally lensed light sources are often shaped into an Einstein ring, due to the elongated shape of the lensing galaxy and the quasar being off-centre, the images form a peculiar cross-shape instead." [1]
While stars can do gravitational lensing, due to both the fact that we can see more galaxies, and they have larger mass, it is easier for it to happen with galaxies.
https://en.wikipedia.org/wiki/Einstein_ring
https://en.wikipedia.org/wiki/Einstein_Cross