For words that only have 3 letters, there is absolutely no 4th letter. As an example with the word “insert”, there aren't any a lot more letters afterwards and thus must be the word "increase". The word “addict” is within the BIP39 wordlist but you might have employed “addi” for https://bookmarkboom.com/story18796790/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39