For words that only have three letters, there is no 4th letter. As an example with the word “include”, there won't be any more letters Later on and therefore have to be the word "increase". The word “addict” is during the BIP39 wordlist however , you might have used “addi” https://bookmarkshome.com/story4446584/bip39-seed-phrase-mnemonics-generator-an-overview