For words that have only 3 letters, there is no 4th letter. For example with the word “insert”, there aren't any a lot more letters Later on and so must be the word "include". The word “addict” is in the BIP39 wordlist however you would've applied “addi” for “addict”. By https://bookmarkalexa.com/story4357476/bip39-seed-phrase-mnemonics-generator-an-overview