For words that only have 3 letters, there is no 4th letter. Such as with the word “add”, there isn't any additional letters afterwards and so needs to be the word "add". The word “addict” is during the BIP39 wordlist however, you would've made use of “addi” for “addict”. You https://getsocialselling.com/story4209901/new-step-by-step-map-for-bip39-english-words