For words that only have 3 letters, there's no 4th letter. As an example with the word “include”, there won't be any a lot more letters afterwards and thus need to be the word "add". The word “addict” is in the BIP39 wordlist however you would've utilised “addi” for “addict”. https://throbsocial.com/story21400204/5-simple-techniques-for-bip39-seed-phrase