For words that only have 3 letters, there's no 4th letter. One example is with the word “insert”, there are no extra letters afterwards and thus should be the word "insert". The word “addict” is while in the BIP39 wordlist but you would've made use of “addi” for “addict”. This https://thesocialcircles.com/story4531827/new-step-by-step-map-for-bip39-english-words