For words that only have three letters, there is no 4th letter. For example with the word “increase”, there isn't any more letters afterwards and therefore have to be the word "include". The word “addict” is in the BIP39 wordlist however, you would have utilised “addi” for “addict”. You may https://socials360.com/story9246359/top-guidelines-of-bip39-word-list-pdf