For words that only have 3 letters, there is absolutely no 4th letter. By way of example with the word “incorporate”, there isn't any extra letters Later on and so must be the word "insert". The word “addict” is during the BIP39 wordlist however you might have used “addi” for https://funny-lists.com/story20309149/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator