For words that only have 3 letters, there isn't any 4th letter. For example with the word “increase”, there aren't any much more letters Later on and thus have to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however, you would've utilised “addi” for “addict”. http://andersonsaeu988.iamarrows.com/from-mnemonics-to-safety-and-security-recognizing-bip39-passphrases-and-their-significance-in-copyright