For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “increase”, there won't be any additional letters afterwards and therefore needs to be the word "add". The word “addict” is within the BIP39 wordlist however , you would've used “addi” for “addict”. https://thebookmarkfree.com/story18933569/the-definitive-guide-to-bip39-seed-phrase