For words that have only three letters, there is not any 4th letter. As an example with the word “insert”, there isn't any much more letters Later on and so have to be the word "incorporate". The word “addict” is within the BIP39 wordlist however , you would've used “addi” https://bookmarkassist.com/story18717789/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator