For words that have only three letters, there is absolutely no 4th letter. For instance with the word “add”, there isn't any additional letters afterwards and so need to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you would have used “addi” for “addict”. Several https://webnowmedia.com/story4420046/the-2-minute-rule-for-bip39-all-words