For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “add”, there isn't any additional letters Later on and so must be the word "increase". The word “addict” is within the BIP39 wordlist but you might have made use of “addi” https://ez-bookmarking.com/story18924116/bip39-phrase-generator-can-be-fun-for-anyone