For words that have only 3 letters, there is absolutely no 4th letter. By way of example with the word “increase”, there aren't any a lot more letters afterwards and thus have to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you would've utilised “addi” https://7prbookmarks.com/story19168967/the-ultimate-guide-to-bip39-seed-phrase-github