For words that only have 3 letters, there is absolutely no 4th letter. As an example with the word “incorporate”, there aren't any far more letters Later on and therefore should be the word "increase". The word “addict” is while in the BIP39 wordlist however , you would've applied “addi” https://wearethelist.com/story20674662/the-2-minute-rule-for-bip39-all-words