For words that only have 3 letters, there's no 4th letter. For instance with the word “increase”, there are no extra letters afterwards and thus must be the word "increase". The word “addict” is from the BIP39 wordlist however, you would have made use of “addi” for “addict”. You should https://bookmarkangaroo.com/story19724946/5-essential-elements-for-bip39-explained