For words that only have 3 letters, there is no 4th letter. One example is with the word “incorporate”, there isn't any additional letters afterwards and so has to be the word "add". The word “addict” is while in the BIP39 wordlist however you would've made use of “addi” for https://thekiwisocial.com/story4243025/invalid-bip39-phrase-things-to-know-before-you-buy