For words that only have three letters, there isn't any 4th letter. For example with the word “include”, there won't be any additional letters afterwards and therefore have to be the word "incorporate". The word “addict” is from the BIP39 wordlist however , you might have made use of “addi” https://mysitesname.com/story8925023/the-greatest-guide-to-seed-phrase-bip39