For words that have only three letters, there isn't any 4th letter. For example with the word “increase”, there aren't any far more letters afterwards and therefore needs to be the word "increase". The word “addict” is in the BIP39 wordlist but you might have applied “addi” for “addict”. You https://altbookmark.com/story20471517/invalid-bip39-phrase-things-to-know-before-you-buy