For words that have only three letters, there is absolutely no 4th letter. As an example with the word “incorporate”, there are no extra letters afterwards and thus must be the word "insert". The word “addict” is within the BIP39 wordlist however , you would have applied “addi” for “addict”. https://wavesocialmedia.com/story4436311/not-known-factual-statements-about-bip39-explained