For words that only have 3 letters, there's no 4th letter. By way of example with the word “insert”, there won't be any additional letters Later on and therefore has to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you would've used “addi” https://getsocialselling.com/story4359843/the-definitive-guide-to-bip39-seed-phrase