There are lots of Ornamental Grasses which are hardy to zones 2, 3 and 4. Most of these will survive your winters in a container.

Compare them with this chart:

http://www.bluestem.ca/grass-comparison-chart.htm

Since your other container plants (the osteospermum and sweet potato vine) are annuals and not winter hardy, I'm not sure why you feel you need an evergreen grass to accompany them :-) Why not pick whatever you like to go with that combo - an annual like purple fountain grass or even one of the common "spike" plants like cordylines/draceanas would work well and last as long as the other plants. IMO, unless you don't mind replanting the container seasonally, combining hardy perennials with annuals just makes for more work and root disturbance to the perennials when you remove/replace the dead annuals. And you'd be surprised how fast the roots of a perennial ornamental grass will fill up a container!

I'm a beginning gardener who is interested in the "cordyline banksii" grass. I live in an apartment and would like to know if it can be kept outside on the balcony during our Southern New Jersey winters? If so, how would I protect it? I also hoped to get another for indoor color. Can this grass be grown as a houseplant? Thank you for your help.

Hi Glo...it isn't a grass, but a sub-tropical tree. It's probably too large to move in and out in a container, as it would need a huge pot.

I suggest you ask your question in a tree forum, or a container forum, or a tropical forum.