For words that have only 3 letters, there's no 4th letter. Such as with the word “increase”, there isn't any additional letters afterwards and therefore have to be the word "increase". The word “addict” is during the BIP39 wordlist however, you would've used “addi” for “addict”. An easy backup method https://exactlybookmarks.com/story18944563/bip39-phrase-generator-can-be-fun-for-anyone