For words that have only 3 letters, there is no 4th letter. For instance with the word “increase”, there isn't any a lot more letters Later on and therefore should be the word "include". The word “addict” is in the BIP39 wordlist but you might have utilised “addi” for “addict”. https://bookmarkinglive.com/story19838907/5-essential-elements-for-bip39-explained