For words that only have 3 letters, there isn't any 4th letter. Such as with the word “add”, there are no more letters Later on and thus have to be the word "add". The word “addict” is in the BIP39 wordlist however, you would've employed “addi” for “addict”. By pursuing https://sociallytraffic.com/story3765575/the-invalid-bip39-phrase-diaries