For words that only have 3 letters, there is absolutely no 4th letter. As an example with the word “insert”, there are no more letters afterwards and thus has to be the word "incorporate". The word “addict” is from the BIP39 wordlist but you would have utilized “addi” for “addict”. https://bookmarkize.com/story19171084/an-unbiased-view-of-bip39-seed-phrase-github