For words that have only 3 letters, there is not any 4th letter. For example with the word “increase”, there are no more letters afterwards and so have to be the word "add". The word “addict” is from the BIP39 wordlist however, you would've applied “addi” for “addict”. BIP39 and https://mysitesname.com/story8566163/a-review-of-bip39-phrase-list