Answer
$\frac{1}{100,000,000}$, unlikely
Work Step by Step
There are 100 possible numbers, so by using the fundamental counting rule we can get the number of possibilities: $100\times100\times100\times100=100,000,000$. We also know that $probability=\frac{number \ of \ good \ outcomes}{number\ of\ all\ outcomes}$, therefore $P=\frac{1}{100,000,000}$. This is a lot of opportuities, so it is very unlikely to open the safe with random tries.
