For words that only have 3 letters, there is no 4th letter. For example with the word “increase”, there isn't any much more letters afterwards and so need to be the word "incorporate". The word “addict” is during the BIP39 wordlist however , you might have applied “addi” for “addict”. https://thebookpage.com/story4417481/the-invalid-bip39-phrase-diaries