For words that have only 3 letters, there isn't any 4th letter. For instance with the word “increase”, there won't be any a lot more letters afterwards and so need to be the word "add". The word “addict” is in the BIP39 wordlist but you would've utilised “addi” for “addict”. https://adsbookmark.com/story19068605/bip39-seed-phrase-fundamentals-explained