For words that only have 3 letters, there is no 4th letter. For instance with the word “increase”, there isn't any additional letters afterwards and thus has to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would've used “addi” for “addict”. In the following https://maximusbookmarks.com/story18992499/invalid-bip39-phrase-things-to-know-before-you-buy