For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “increase”, there won't be any much more letters afterwards and therefore should be the word "insert". The word “addict” is inside the BIP39 wordlist but you might have employed “addi” for https://bookmarkstown.com/story19024623/the-2-minute-rule-for-bip39-all-words