For words that only have 3 letters, there is no 4th letter. Such as with the word “increase”, there are no much more letters Later on and so have to be the word "include". The word “addict” is within the BIP39 wordlist however you would've utilised “addi” for “addict”. This https://classifylist.com/story20776292/the-basic-principles-of-bip39-passphrase-generator