For words that only have three letters, there is no 4th letter. For example with the word “increase”, there isn't any additional letters Later on and therefore should be the word "add". The word “addict” is while in the BIP39 wordlist however you would have utilised “addi” for “addict”. Tips https://bookmarkmoz.com/story19065470/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting