For words that have only three letters, there isn't any 4th letter. For example with the word “insert”, there aren't any more letters Later on and therefore have to be the word "add". The word “addict” is in the BIP39 wordlist however , you might have used “addi” for “addict”. https://socialtechnet.com/story4252714/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator