For words that only have three letters, there isn't any 4th letter. By way of example with the word “increase”, there isn't any more letters afterwards and thus has to be the word "increase". The word “addict” is during the BIP39 wordlist however you would have employed “addi” for “addict”. https://ragingbookmarks.com/story18806140/the-invalid-bip39-phrase-diaries