For words that have only three letters, there is not any 4th letter. As an example with the word “increase”, there isn't any much more letters Later on and therefore should be the word "insert". The word “addict” is while in the BIP39 wordlist however , you would have used https://captainbookmark.com/story18984613/an-unbiased-view-of-bip39-seed-phrase-github