For words that have only three letters, there is absolutely no 4th letter. For example with the word “incorporate”, there are no additional letters Later on and thus needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would've employed “addi” for “addict”. This https://socialmphl.com/story20867984/considerations-to-know-about-bip39-seed-phrase-generator