For words that only have 3 letters, there is not any 4th letter. For example with the word “increase”, there are no extra letters Later on and therefore have to be the word "insert". The word “addict” is while in the BIP39 wordlist however, you would've used “addi” for “addict”. https://allkindsofsocial.com/story4174514/top-latest-five-bip39-words-urban-news