For words that have only 3 letters, there is absolutely no 4th letter. For example with the word “include”, there are no extra letters afterwards and thus have to be the word "add". The word “addict” is from the BIP39 wordlist however you might have made use of “addi” for https://e-bookmarks.com/story4469043/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator