For words that have only 3 letters, there is no 4th letter. One example is with the word “incorporate”, there are no extra letters afterwards and so has to be the word "add". The word “addict” is while in the BIP39 wordlist but you might have utilized “addi” for “addict”. https://bookmarkize.com/story18828859/bip39-seed-phrase-mnemonics-generator-an-overview