For words that have only three letters, there isn't any 4th letter. By way of example with the word “increase”, there aren't any more letters afterwards and thus have to be the word "increase". The word “addict” is during the BIP39 wordlist however you might have utilised “addi” for “addict”. https://bookmarkdistrict.com/story19355454/considerations-to-know-about-bip39-word-list-pdf