For words that have only three letters, there is not any 4th letter. As an example with the word “include”, there are no more letters Later on and therefore must be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you might have utilised “addi” for “addict”. https://tornadosocial.com/story4567186/the-invalid-bip39-phrase-diaries