For words that have only three letters, there isn't any 4th letter. Such as with the word “insert”, there are no far more letters Later on and therefore has to be the word "increase". The word “addict” is within the BIP39 wordlist however , you would've utilised “addi” for “addict”. https://bookmarksden.com/story19022689/new-step-by-step-map-for-bip39-english-words