For words that have only three letters, there is not any 4th letter. For example with the word “add”, there are no more letters afterwards and thus should be the word "include". The word “addict” is inside the BIP39 wordlist but you would have utilized “addi” for “addict”. This informative https://ragingbookmarks.com/story18805373/bip39-seed-phrase-fundamentals-explained