For words that only have three letters, there is no 4th letter. For example with the word “increase”, there isn't any extra letters afterwards and so should be the word "incorporate". The word “addict” is during the BIP39 wordlist however, you might have made use of “addi” for “addict”. - https://telebookmarks.com/story9087129/new-step-by-step-map-for-bip39-english-words