For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “add”, there isn't any additional letters Later on and therefore must be the word "increase". The word “addict” is within the BIP39 wordlist but you would've employed “addi” for “addict”. https://greatbookmarking.com/story19064756/bip39-phrase-generator-can-be-fun-for-anyone