For words that only have three letters, there isn't any 4th letter. For example with the word “increase”, there aren't any more letters Later on and therefore must be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would have utilised “addi” for “addict”. This https://bookmarksurl.com/story4272965/the-5-second-trick-for-ledger-seed-phrase-bip39