For words that only have 3 letters, there is absolutely no 4th letter. Such as with the word “insert”, there aren't any a lot more letters afterwards and thus should be the word "incorporate". The word “addict” is during the BIP39 wordlist however, you might have applied “addi” for “addict”. https://nowbookmarks.com/story19055288/the-2-minute-rule-for-bip39-all-words