SuperTokens is an open source authentication solution offering features like: Different types of login: Email / password, Passwordless (OTP or Magic link based).

SuperTokens.com

https://supertokens.com/docs/passwordless/common-customizations/sign-in-and-up/change-otp-format#:~:text=By%20default%2C%20the%20generated%20OTP,by%20providing%20the%20getCustomUserInputCode%20function.

I am checking this documentation 
Could not find how to change length of otp from 6 to 4

thenh return otp of length 4 from the func