For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “incorporate”, there isn't any far more letters afterwards and thus needs to be the word "add". The word “addict” is within the BIP39 wordlist however you would've employed “addi” for “addict”. https://sitesrow.com/story8620223/the-definitive-guide-to-bip39-seed-phrase