For words that only have three letters, there is not any 4th letter. As an example with the word “increase”, there are no more letters afterwards and therefore should be the word "add". The word “addict” is while in the BIP39 wordlist however, you might have used “addi” for “addict”. https://natural-bookmark.com/story18799740/invalid-bip39-phrase-things-to-know-before-you-buy