For words that have only 3 letters, there is absolutely no 4th letter. One example is with the word “include”, there won't be any additional letters afterwards and thus have to be the word "increase". The word “addict” is inside the BIP39 wordlist but you might have employed “addi” for https://thekiwisocial.com/story4488888/the-ultimate-guide-to-bip39-seed-phrase-github