For words that have only 3 letters, there is not any 4th letter. As an example with the word “increase”, there aren't any extra letters afterwards and so need to be the word "add". The word “addict” is inside the BIP39 wordlist however you would have utilized “addi” for “addict”. https://bookmarkingace.com/story18842091/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator