For words that have only 3 letters, there isn't any 4th letter. As an example with the word “increase”, there aren't any more letters Later on and therefore have to be the word "incorporate". The word “addict” is in the BIP39 wordlist but you might have made use of “addi” https://thesocialroi.com/story9280510/bip39-seed-phrase-mnemonics-generator-an-overview