For words that have only three letters, there's no 4th letter. For example with the word “insert”, there won't be any a lot more letters afterwards and so have to be the word "add". The word “addict” is in the BIP39 wordlist but you would've applied “addi” for “addict”. In https://bookmarkedblog.com/story19577934/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator