For words that have only 3 letters, there is absolutely no 4th letter. For instance with the word “increase”, there isn't any extra letters Later on and so needs to be the word "incorporate". The word “addict” is within the BIP39 wordlist but you would've utilised “addi” for “addict”. BIP39 https://classifylist.com/story20642597/facts-about-bip39-seed-phrase-github-revealed