For words that only have three letters, there isn't a 4th letter. One example is with the word “insert”, there won't be any more letters afterwards and so have to be the word "increase". The word “addict” is inside the BIP39 wordlist however, you might have used “addi” for “addict”. https://wearethelist.com/story20917732/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator