For words that have only 3 letters, there is no 4th letter. For example with the word “increase”, there aren't any far more letters afterwards and thus must be the word "increase". The word “addict” is inside the BIP39 wordlist however, you would have made use of “addi” for “addict”. https://bookmarkproduct.com/story18907196/examine-this-report-on-bip39-mnemonic-phrase-tool