For words that only have three letters, there is absolutely no 4th letter. For instance with the word “insert”, there won't be any a lot more letters afterwards and thus should be the word "add". The word “addict” is during the BIP39 wordlist but you would've utilized “addi” for “addict”. https://mysitesname.com/story8752963/bip39-seed-phrase-fundamentals-explained