For words that have only 3 letters, there isn't a 4th letter. By way of example with the word “add”, there aren't any much more letters Later on and so should be the word "add". The word “addict” is within the BIP39 wordlist but you would've used “addi” for “addict”. https://socialmphl.com/story20803928/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator