For words that have only 3 letters, there is no 4th letter. As an example with the word “add”, there won't be any extra letters afterwards and so has to be the word "increase". The word “addict” is while in the BIP39 wordlist but you might have made use of https://mixbookmark.com/story4396978/ledger-seed-phrase-bip39-no-further-a-mystery