After all the years of nannies I've had, I would part ways. If it was one, easily corrected thing, like cleaning up the bottles and lunch dishes, I'd say to sit down, explain expectations, and give her a chance to solve the problem.
But if you feel like you're going to have to teach her how to manage a 4-year-old and an infant simultaneously, that's too much to fix.
My answer changes if your new baby has colic, or is otherwise difficult to put down for even a few minutes.