For words that have only three letters, there is absolutely no 4th letter. For instance with the word “increase”, there aren't any extra letters Later on and therefore need to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however you would've utilized “addi” for “addict”. https://socialwebleads.com/story4353561/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition