For words that only have 3 letters, there isn't any 4th letter. One example is with the word “insert”, there won't be any additional letters afterwards and therefore should be the word "add". The word “addict” is during the BIP39 wordlist however you would've made use of “addi” for “addict”. https://tvsocialnews.com/story4995284/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator