For words that have only 3 letters, there is no 4th letter. Such as with the word “incorporate”, there won't be any far more letters afterwards and so needs to be the word "insert". The word “addict” is inside the BIP39 wordlist but you would've utilized “addi” for “addict”. Standard https://bookmarkjourney.com/story18842857/bip39-seed-phrase-fundamentals-explained