Your subplot only has 4x1 subfigures ie 4 with indexes 1,2,3,4
Your index vector i on the other hand tries to access 2,4,6,8.
Two of those indexes (6,8) don't exist and exceed the number of subplots.
To quickly fix the problem just change the for argument to i=1:4 and add a *2 to the y function
i = k*2;