For words that have only 3 letters, there's no 4th letter. One example is with the word “insert”, there won't be any more letters afterwards and therefore need to be the word "incorporate". The word “addict” is in the BIP39 wordlist however , you would've applied “addi” for “addict”. Artwork https://thesocialintro.com/story4410253/the-definitive-guide-to-bip39-seed-phrase