For words that only have 3 letters, there is absolutely no 4th letter. As an example with the word “incorporate”, there won't be any extra letters afterwards and so must be the word "incorporate". The word “addict” is from the BIP39 wordlist but you might have used “addi” for “addict”. https://minibookmarks.com/story18833084/not-known-facts-about-bip39-english-words