For words that have only 3 letters, there is no 4th letter. Such as with the word “increase”, there aren't any more letters afterwards and thus should be the word "add". The word “addict” is from the BIP39 wordlist however you would've utilised “addi” for “addict”. By next these guidelines, https://johsocial.com/story9183149/considerations-to-know-about-bip39-seed-phrase-generator