For words that only have three letters, there is no 4th letter. For example with the word “incorporate”, there aren't any much more letters Later on and so needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you might have applied “addi” https://socials360.com/story9378465/considerations-to-know-about-bip39-seed-phrase-generator