For words that only have three letters, there is no 4th letter. By way of example with the word “increase”, there are no far more letters afterwards and thus needs to be the word "include". The word “addict” is inside the BIP39 wordlist however , you would have used “addi” https://getsocialselling.com/story4397616/the-ultimate-guide-to-bip39-seed-phrase-github