Generate all combinations of 4 digits passcode/pin
each digit starts from 0 and ends on 9
total combinations would be 10,000: 10*4
0 0 0 0
0 0 0 1
. . . .
0 0 0 9
0 0 1 0
0 0 1 1
. . . .
0 0 1 9
0 0 2 0
. . . .
0 1 0 0
. . . .
1 0 0 0
. . . .
9 9 9 9