For words that only have three letters, there is absolutely no 4th letter. Such as with the word “increase”, there are no extra letters afterwards and therefore should be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would've used “addi” for “addict”. In this https://bookmarkpressure.com/story18739765/bip39-seed-phrase-fundamentals-explained