0-36 equals 37 total numbers.
So each number in the combination could be one of 37 numbers.
1st number can be 1 of 37
2nd number is 1 of 37
3rd number is 1 of 37
4th number is 1 of 37
Multiply each possibility by each other.
Total number of combinations is 37 x 37 x 37 x 37 = 1,874,161