For words that only have 3 letters, there is absolutely no 4th letter. Such as with the word “insert”, there isn't any far more letters afterwards and so has to be the word "incorporate". The word “addict” is from the BIP39 wordlist however you would have applied “addi” for “addict”. https://thebookmarkage.com/story19121611/new-step-by-step-map-for-ledger-seed-phrase-bip39