For words that only have three letters, there is absolutely no 4th letter. As an example with the word “increase”, there are no more letters Later on and so need to be the word "incorporate". The word “addict” is in the BIP39 wordlist but you might have made use of http://collinmmss433.lucialpiazzale.com/opening-the-power-of-bip39-a-comprehensive-check-out-word-lists-and-secure-pocketbooks