For words that have only 3 letters, there is no 4th letter. Such as with the word “add”, there are no much more letters Later on and so should be the word "increase". The word “addict” is from the BIP39 wordlist however, you would've made use of “addi” for “addict”. https://bookmarkbirth.com/story18982687/new-step-by-step-map-for-bip39-english-words