For words that only have three letters, there isn't a 4th letter. Such as with the word “incorporate”, there are no extra letters afterwards and thus must be the word "insert". The word “addict” is inside the BIP39 wordlist but you would've utilized “addi” for “addict”. BIP39 and forty four, https://monobookmarks.com/story18770433/ledger-seed-phrase-bip39-no-further-a-mystery