For words that only have three letters, there is absolutely no 4th letter. For instance with the word “increase”, there are no more letters afterwards and thus needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however you would've applied “addi” for “addict”. BIP39 and https://mysocialport.com/story4256337/examine-this-report-on-bip39-mnemonic-phrase-tool