For words that have only 3 letters, there is no 4th letter. Such as with the word “insert”, there isn't any a lot more letters Later on and therefore has to be the word "include". The word “addict” is in the BIP39 wordlist however, you would've utilised “addi” for “addict”. https://thesocialdelight.com/story4344054/5-simple-techniques-for-bip39-word-list-pdf