For words that only have 3 letters, there is absolutely no 4th letter. By way of example with the word “increase”, there are no extra letters Later on and thus must be the word "insert". The word “addict” is inside the BIP39 wordlist however you would've employed “addi” for “addict”. https://socialmediastore.net/story19410434/top-latest-five-bip39-words-urban-news