The work of a student to solve a set of equations is shown:

equation a: y = 4 – 2z
equation b: 6y = 2 – 4z

step 1: –6(y) = –6(4 – 2z) [equation a is multiplied by –6.]
6y = 2 – 4z [equation b]
step 2: 6y = 4 – 2z [equation a in step 1 is simplified.]
6y = 2 – 4z [equation b]
step 3: 0 = 6 – 6z [equations in step 2 are added.]
step 4: 6z = 6
step 5: z = 1

in which step did the student first make an error?
step 4
step 3
step 2
step 1