For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “increase”, there won't be any much more letters afterwards and thus have to be the word "include". The word “addict” is from the BIP39 wordlist but you might have made use https://e-bookmarks.com/story4454055/facts-about-bip39-seed-phrase-word-list-revealed