For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “add”, there won't be any more letters Later on and thus must be the word "add". The word “addict” is within the BIP39 wordlist however , you would've utilised “addi” for “addict”. https://tealbookmarks.com/story18815607/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting