For words that only have three letters, there isn't any 4th letter. By way of example with the word “increase”, there isn't any a lot more letters afterwards and so must be the word "add". The word “addict” is in the BIP39 wordlist however , you might have utilised “addi” https://thebookmarkking.com/story18768668/a-review-of-bip39-phrase-list