For words that only have three letters, there isn't any 4th letter. For instance with the word “increase”, there aren't any more letters Later on and so needs to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however, you might have used “addi” for “addict”. In https://thekiwisocial.com/story4289457/the-5-second-trick-for-ledger-seed-phrase-bip39