First things first, the fundamental counting principle (FCP) tells you that if there is m possible ways for an event to occur and n possible ways for another event to occur then there are n*m possible ways for both events to occur. How is this related to question? Well, in this problem you need four events to occur: the correct selection of the 4 digits in the code. The number of outcomes in this situations would be the number of possible codes. According to the FCP we need to multiple 4 numbers. Each being the different numbers we can use in each digit.
Each digit can be a number between 0 and 9 so there are 10 possible outcomes for each digit (remember to count the 0). Following the FCP we have that the total number of outcomes is 10*10*10*10 = 10000.