For words that have only 3 letters, there's no 4th letter. One example is with the word “increase”, there are no much more letters afterwards and so should be the word "include". The word “addict” is inside the BIP39 wordlist however , you would've used “addi” for “addict”. An easy https://webnowmedia.com/story4228487/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting