For words that have only three letters, there is absolutely no 4th letter. As an example with the word “increase”, there isn't any more letters Later on and thus has to be the word "incorporate". The word “addict” is within the BIP39 wordlist but you might have applied “addi” for https://socialexpresions.com/story4368803/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase