For words that only have three letters, there is absolutely no 4th letter. For example with the word “add”, there isn't any more letters afterwards and therefore need to be the word "include". The word “addict” is from the BIP39 wordlist however, you would have used “addi” for “addict”. The https://checkbookmarks.com/story4344863/an-unbiased-view-of-bip39-seed-phrase-github