For words that only have 3 letters, there is no 4th letter. As an example with the word “insert”, there won't be any much more letters Later on and thus needs to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however , you would have utilised https://socialwoot.com/story20390577/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39