For words that have only three letters, there's no 4th letter. For instance with the word “increase”, there aren't any much more letters afterwards and so have to be the word "add". The word “addict” is in the BIP39 wordlist but you might have made use of “addi” for “addict”. https://whitebookmarks.com/story18912691/the-definitive-guide-to-bip39-seed-phrase