Hi NB,
Please take out the amount you entered in "Other Employment Income". That should be nil in your situation. The only place you need to enter your self employment income (and $4K expense) is in Business Expert Interview area.
To answer your other question, yes you are eligible for the Working Income Tax Benefit (WITB). The federal government encourages individuals to join work force, and if income is under a certain threshold you get money back as calculated in WITB worksheet. It is not a mistake and you don't need to pay this refund back, as long as you can prove your income & expenses.
(...and I must commend your style to explain in detail. Here in the forum we usually get high level generic questions with little detail. Thorough statement like yours makes our response easier)
