For words that have only 3 letters, there's no 4th letter. One example is with the word “increase”, there are no much more letters Later on and thus need to be the word "insert". The word “addict” is in the BIP39 wordlist however, you would've used “addi” for “addict”. If https://iwanttobookmark.com/story19723502/bip39-seed-phrase-mnemonics-generator-an-overview