For words that only have 3 letters, there is no 4th letter. For example with the word “increase”, there isn't any additional letters afterwards and so needs to be the word "insert". The word “addict” is inside the BIP39 wordlist however you would have made use of “addi” for “addict”. https://tornadosocial.com/story5028878/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition