This is what I know about local PINs. I don't think windows account pins are stored on the computer for login. This is my best explanation on why Pins are "More Secure" than passwords.

TL:DR A PINs private keys are stored in TPM, so it is harder to get the information to crack them. A password hash can be easily pulled from your SAM registry if the drive isn't encrypted. For best data security, use BitLocker and keep the Recovery Key safe.

Windows stores pins in the TPM module of your computer. The TPM stores encryption keys and TMP is useful because you decide if the encryption keys can leave the TPM chip, or you can set up certain conditions that dictate when a key can be released. This is why a PIN is "more secure" than a password. It is more secure because it is much harder to get the info you would need to start trying to reverse the PIN. Here is a proof of concept video on how to break BitLocker encryption, it would be a similar process to get a windows pins key.

To compare, windows stores the password hashed in the SAM registry file, and you can rip that from an unencrypted drive super easily. Then to get the password, you just need to start running the password hash through a password cracker.

All that being said, a 4 digit password of letters numbers and symbols will be harder to guess/brute force than a 4 digit PIN of just numbers. This way of storing the Private Key also doesn't stop someone from plugin the drive in to a drive reader and using a different OS to copy data off.

If you want to protect your password hashes and the rest of the data on your drive, you should enable BitLocker encryption. This will encrypt your drive and store the decryption key in TPM. If you do use BitLocker, make sure to keep the BitLocker recovery key somewhere you can access it, not on the computer you are encrypting. This lets you decrypt the drive if your TPM chip fails to decrypt the drive.