For words that have only 3 letters, there is not any 4th letter. As an example with the word “insert”, there won't be any much more letters afterwards and so needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist but you might have used “addi” https://bookmark-rss.com/story18659684/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase