For words that only have three letters, there isn't a 4th letter. For instance with the word “increase”, there are no far more letters afterwards and so should be the word "incorporate". The word “addict” is inside the BIP39 wordlist however, you would have made use of “addi” for “addict”. https://webnowmedia.com/story4344241/the-ultimate-guide-to-bip39-seed-phrase-github