For words that only have three letters, there isn't any 4th letter. One example is with the word “increase”, there won't be any extra letters afterwards and so need to be the word "insert". The word “addict” is within the BIP39 wordlist however, you would have used “addi” for “addict”. https://hotbookmarkings.com/story18864221/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase