For words that only have three letters, there is no 4th letter. As an example with the word “add”, there isn't any more letters Later on and so should be the word "include". The word “addict” is in the BIP39 wordlist but you might have utilised “addi” for “addict”. You https://dailybookmarkhit.com/story19673120/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition