For words that only have 3 letters, there isn't a 4th letter. One example is with the word “increase”, there are no more letters Later on and thus have to be the word "include". The word “addict” is inside the BIP39 wordlist however, you would've employed “addi” for “addict”. BIP39 https://socialrus.com/story18380839/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator