For words that have only three letters, there isn't a 4th letter. As an example with the word “insert”, there isn't any much more letters Later on and therefore need to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you would've applied “addi” for “addict”. https://bookmarkpressure.com/story18955497/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator