For words that have only three letters, there is absolutely no 4th letter. One example is with the word “incorporate”, there are no more letters Later on and thus have to be the word "add". The word “addict” is inside the BIP39 wordlist however, you would've employed “addi” for “addict”. https://wildbookmarks.com/story19070916/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39