For words that have only 3 letters, there is not any 4th letter. One example is with the word “incorporate”, there are no additional letters afterwards and thus should be the word "increase". The word “addict” is while in the BIP39 wordlist however, you might have utilised “addi” for “addict”. https://gatherbookmarks.com/story19688242/bip39-seed-phrase-mnemonics-generator-an-overview