For words that only have three letters, there is absolutely no 4th letter. By way of example with the word “incorporate”, there isn't any additional letters afterwards and so needs to be the word "insert". The word “addict” is in the BIP39 wordlist however , you might have employed “addi” https://bookmarkshome.com/story4625729/bip39-seed-phrase-mnemonics-generator-an-overview