For words that only have 3 letters, there is no 4th letter. By way of example with the word “add”, there isn't any additional letters Later on and so must be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you would have made use of “addi” https://socialeweb.com/story4420516/the-2-minute-rule-for-bip39-all-words