For words that have only 3 letters, there's no 4th letter. As an example with the word “insert”, there aren't any far more letters afterwards and thus must be the word "incorporate". The word “addict” is while in the BIP39 wordlist however , you might have employed “addi” for “addict”. https://bookmarkoffire.com/story18948950/invalid-bip39-phrase-things-to-know-before-you-buy