For words that only have three letters, there is absolutely no 4th letter. As an example with the word “increase”, there isn't any a lot more letters Later on and thus must be the word "increase". The word “addict” is inside the BIP39 wordlist but you might have used “addi” https://indexedbookmarks.com/story18839486/facts-about-bip39-seed-phrase-github-revealed