Discipline for a 4 year old?

Okay ladies, I really need help.  I'm babysitting a boy and a girl, the girl (Cathrin) and the boy (Justin) were coloring.  Justin asked me to color with him, so I did.  Cathrin got upset, and pitched a fit and cried because I was coloring with them.  She proceeded to push me, hit me, and even tried to push me down the stairs.  She has pushed Justin down the Stairs a while back as well.  I didn't fall but I stumbled, and then she made Justin play with her.  When they were playing just the two of them, she says "Sorry Emily, I guess Justin wants to play with me and not you." I didn't know what to say, I was speechless.  She had a bratty look on her face, like she was challenging me.  I honestly don't know what to do.  How can I discipline a child like her?  Time out just doesn't seem to work.