For words that only have three letters, there is no 4th letter. Such as with the word “add”, there isn't any a lot more letters afterwards and therefore must be the word "incorporate". The word “addict” is in the BIP39 wordlist however you might have applied “addi” for “addict”. Mnemonic https://bookmarkspecial.com/story19032456/5-essential-elements-for-bip39-explained