How many 4 number combinations can made from four numbers?

Like 1235 or 8067?

If you allow leading zeros, then 10 000.

If you disallow leading zeros, then 9 000.

* * * * *

Those are the numbers of PERMUTATIONS, not COMBINATIONS.

In combinations the order does not matter, so 1234 = 4312 = 2314 etc.

The number of combinations is (10*9*8*7)/(4*3*2*1) = 210.

The question of leading 0s does not arise because there is no order to the set of numbers in the combination so there is no "leading".