Answer
$10000$
Work Step by Step
The fundamental counting principle says that if there are $|A|$ outcomes of the event $A$ and $|B|$ of the event $|B|$, then the number of outcomes of event A and event B combined is $|A|\cdot|B|$. This can easily be extended to $n$ events.
Here the first three digits are locked, thus there is $1$ possibility for these, however for the rest, there are $10$ possibilities for every digit.
Using Fundamental Principle of Counting the total number of possibilities: $1\cdot10^4=10000$