For words that have only 3 letters, there isn't any 4th letter. As an example with the word “increase”, there aren't any more letters Later on and thus need to be the word "insert". The word “addict” is within the BIP39 wordlist but you might have employed “addi” for “addict”. https://mysocialguides.com/story4212585/a-secret-weapon-for-bip39-words-to-numbers