For words that only have three letters, there is absolutely no 4th letter. For example with the word “increase”, there are no much more letters afterwards and thus must be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would have applied “addi” for “addict”. To http://rylanmecp341.wpsuo.com/from-words-to-wide-range-exactly-how-to-efficiently-make-use-of-bip39-passphrases-for-your-copyright-pocketbooks