For words that only have 3 letters, there's no 4th letter. By way of example with the word “add”, there are no far more letters afterwards and thus needs to be the word "add". The word “addict” is during the BIP39 wordlist however, you might have utilized “addi” for “addict”. https://socialbookmarkgs.com/story19071313/an-unbiased-view-of-bip39-seed-phrase-github