For words that only have three letters, there is absolutely no 4th letter. As an example with the word “include”, there isn't any additional letters afterwards and so has to be the word "increase". The word “addict” is from the BIP39 wordlist however you might have used “addi” for “addict”. https://mnobookmarks.com/story18788162/little-known-facts-about-bip39-seed-phrase-list