For words that have only three letters, there is absolutely no 4th letter. As an example with the word “incorporate”, there are no a lot more letters Later on and so must be the word "incorporate". The word “addict” is within the BIP39 wordlist but you would have used “addi” https://socialtechnet.com/story4272977/the-2-minute-rule-for-bip39-all-words