For words that only have 3 letters, there isn't any 4th letter. One example is with the word “incorporate”, there aren't any much more letters Later on and so has to be the word "add". The word “addict” is from the BIP39 wordlist but you might have made use of https://thebookmarkking.com/story18915896/bip39-seed-phrase-mnemonics-generator-an-overview