mifinity でカードを確認するためのコードが見つかりません

Where do I find the 4-digit code to verify my card?

donde encuentro el codigo de 4 digitos para verificar mi tarjeta

Hello, it depends on the card and most probably on the requirement given.

So, which numbers are you supposed to provide?

I find the manual for you:

https://support.mifinity.com/hc/en-gb/articles/4407912132113-Verifying-your-Credit-Debit-Cards-

It says: "Enter the 4-digit verification code obtained from your card statement and click on "Submit"

Providing the pin is correct, your payment method will be verified. For any issues with incorrect 4 digit verification code displayed on your card statement or no 4 digit code displayed on your card statement please contact directly your card issuing bank.

You must find the letter from the bank, I'd say.