For words that have only three letters, there is absolutely no 4th letter. One example is with the word “insert”, there won't be any more letters Later on and thus needs to be the word "add". The word “addict” is from the BIP39 wordlist however , you would have applied https://growthbookmarks.com/story19549250/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator