For words that have only three letters, there isn't any 4th letter. For instance with the word “increase”, there isn't any much more letters afterwards and so should be the word "add". The word “addict” is in the BIP39 wordlist however , you would have used “addi” for “addict”. You https://altbookmark.com/story20660694/the-definitive-guide-to-bip39-seed-phrase