For words that only have 3 letters, there is absolutely no 4th letter. Such as with the word “add”, there isn't any additional letters afterwards and thus must be the word "incorporate". The word “addict” is while in the BIP39 wordlist however you would have used “addi” for “addict”. A https://reallivesocial.com/story4714391/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase