For words that only have 3 letters, there isn't a 4th letter. By way of example with the word “include”, there isn't any additional letters afterwards and therefore must be the word "insert". The word “addict” is from the BIP39 wordlist however, you would've made use of “addi” for “addict”. https://thesocialcircles.com/story4515966/considerations-to-know-about-bip39-seed-phrase-generator