Even IF you take them to court and win there is no guarantee that you will get paid. Carefully consider the time, effort and cost that it will take to take them to small claims court (losing hours / days of work, paying court costs etc) before you decide to go ahead. Even if you win all that will happen is a judgement will be placed against them, and you still may not ever get your money.
Check the forum, some NSA's have given some very creative ways to twist the arm of a non paying companies that may work better for you and taking them to court.
Also, talk to your accountant and write the $400 off as bad debt if you don't get paid.