For words that only have 3 letters, there isn't any 4th letter. As an example with the word “increase”, there isn't any additional letters afterwards and so must be the word "include". The word “addict” is within the BIP39 wordlist but you would've applied “addi” for “addict”. This additional complexity https://bookmarkextent.com/story20675814/top-latest-five-bip39-words-urban-news