For words that have only 3 letters, there is no 4th letter. By way of example with the word “incorporate”, there won't be any more letters afterwards and thus should be the word "incorporate". The word “addict” is while in the BIP39 wordlist but you would've employed “addi” for “addict”. https://bookmarkshome.com/story4671777/little-known-facts-about-bip39-words