In this case since repetition is allowed as well as starting with zero, the number of 4 digits chosen among the 10 (from 0 to 9):
the 1st digit can be selected in 10 ways
the 2ndt digit can be selected in 10 ways
the 3rd digit can be selected in 10 ways
the 4th digit can be selected in 10 ways
Or 10 x 10 10 x10 = 10⁴ = 10,000 Arrangements