For words that only have three letters, there is no 4th letter. For instance with the word “increase”, there aren't any more letters afterwards and thus have to be the word "add". The word “addict” is from the BIP39 wordlist but you would have utilized “addi” for “addict”. Should you https://bookmarkleader.com/story18930657/the-2-minute-rule-for-bip39-all-words