With only 4 digits, 10,000 possible choices, it would take a computer a short time to determine your pin, assuming they knew the salt. If they did not know the salt, I assume it would be slightly longer depending on how big the salt was.

I had not really thought about this problem, of the PIN being so short, but I assume they work on the idea of: some thing you own (the card), and something you know (the PIN) to provide security.