For words that have only 3 letters, there's no 4th letter. As an example with the word “add”, there isn't any a lot more letters afterwards and so has to be the word "increase". The word “addict” is in the BIP39 wordlist however , you might have used “addi” for https://reallivesocial.com/story4396157/new-step-by-step-map-for-bip39-english-words