For words that only have three letters, there is absolutely no 4th letter. For instance with the word “increase”, there aren't any more letters Later on and so have to be the word "add". The word “addict” is during the BIP39 wordlist however you would've utilized “addi” for “addict”. This https://getsocialsource.com/story4264648/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator