For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “add”, there aren't any more letters afterwards and so need to be the word "add". The word “addict” is during the BIP39 wordlist but you would have applied “addi” for https://one-bookmark.com/story18965223/an-unbiased-view-of-bip39-seed-phrase-github