For words that only have 3 letters, there is no 4th letter. As an example with the word “increase”, there won't be any additional letters Later on and so needs to be the word "add". The word “addict” is within the BIP39 wordlist however you would've employed “addi” for “addict”. https://free-bookmarking.com/story18923651/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool