For words that have only 3 letters, there isn't any 4th letter. For example with the word “insert”, there are no more letters Later on and so has to be the word "increase". The word “addict” is from the BIP39 wordlist however , you would have utilised “addi” for “addict”. https://guidemysocial.com/story4258266/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition