Revisiting your ipad’s security, what happens if the order in which the digits are entered does not matter? If so, then you would be picking a combination of 4 digits without replacement from a group of 10 digits. Namely,
\begin{align*}
\frac{10!}{4!6!} & = \frac{10 \times 9 \times 8 \times 7 \times 6!}{4 \times 3 \times 2 \times 1 \times 6!}\\
& = \frac{10 \times 9 \times 8 \times 7}{4 \times 3 \times 2 \times 1}\\
& = \frac{5040}{24}\\
& = 210.
\end{align*}
Notice that the total number of options is much smaller when order does not matter.