Which of the following systems of equations has no solution?

Which of the following systems of equations has no solution?
1. { y = 2x - 4 \ y = 2x + 4 }
2. { y = 4x - 5 \ 2y = 8x - 10 }
3. { y = 1/2x - 6 \ y = -1/2x - 6 }
4. { y = 3x - 2 \ y = 2x - 3 }