For words that only have three letters, there is absolutely no 4th letter. Such as with the word “incorporate”, there won't be any much more letters afterwards and so must be the word "add". The word “addict” is from the BIP39 wordlist however, you would have made use of “addi” https://yourbookmarklist.com/story18987715/the-ultimate-guide-to-bip39-seed-phrase-github