For words that have only three letters, there's no 4th letter. As an example with the word “incorporate”, there are no additional letters afterwards and therefore has to be the word "add". The word “addict” is while in the BIP39 wordlist but you might have made use of “addi” for https://reallivesocial.com/story4368495/5-simple-techniques-for-bip39-word-list-pdf