For words that only have 3 letters, there isn't any 4th letter. One example is with the word “include”, there isn't any much more letters Later on and therefore has to be the word "incorporate". The word “addict” is within the BIP39 wordlist but you would've applied “addi” for “addict”. https://dftsocial.com/story20405788/top-latest-five-bip39-words-urban-news