For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “add”, there are no more letters Later on and so needs to be the word "insert". The word “addict” is in the BIP39 wordlist but you would've employed “addi” for “addict”. https://thebookmarkfree.com/story19047683/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator