Is it possible to increase the encryption security of a short key? If I encrypt a file with a short 4 digit pin is there any way to make it more secure. I've thought about hashing the 4 digit pin and using the result as the encryption key. I think this would work as long as the attacker didn't know that I was using this strategy but if they did they could just try the 10^4 hashing of the possible pins.
The hash idea isn't great. See Kerckhoffs's principle. The underlying secret is still the four digit PIN, which is only $4 \times -\log_2(10) = 13$ bits of security. And don't we all pick '1234' anyway?
You can use a key derivation function like Argon2 (there are others). That slows down hackers attempts at guessing your PIN. You could aim for something like one second on typical hardware. Still it could be brute forced after a mean time of an hour and a quarter on that same kit though.
Security could be further enhanced with an $n$ tries and you're locked out policy if there is a server involved with conditional logic. This would be implemented by also using the derived encryption key to encrypt a series of four check digits, say all zero. If they do not decrypt to all zero, it's the wrong PIN, so try again until lock out. This is how your ATM card behaves.
And a further further method would be to have an increasing/exponential delay between attempts. This would depend on your customers' acceptance though.

A longer pin is an option if the business allows it. Windows login PINs are six digits.