For words that only have three letters, there is no 4th letter. As an example with the word “add”, there isn't any far more letters afterwards and therefore should be the word "increase". The word “addict” is during the BIP39 wordlist but you would have utilised “addi” for “addict”. BIP39 https://bookmarkshome.com/story4436535/not-known-facts-about-bip39-english-words