4 minutes
If you started at the same time, and both your sister and you stay at the same constant rate, you would have raked 8 more squared meters than her after 4 minutes.
Zero minutes:You rated 0, she raked 0
One minute:You raked 4, she raked 2
Two minutes:You raked 8, she raked 4
Three minutes:You raked 12 she raked 6
Four minutes:You raked 16 she raked 8