For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “incorporate”, there aren't any a lot more letters Later on and thus have to be the word "add". The word “addict” is during the BIP39 wordlist however, you would have used https://bookmarkuse.com/story18646391/an-unbiased-view-of-bip39-seed-phrase-github