For words that only have 3 letters, there is no 4th letter. For instance with the word “increase”, there won't be any a lot more letters afterwards and therefore should be the word "increase". The word “addict” is inside the BIP39 wordlist but you would have utilised “addi” for “addict”. https://mysitesname.com/story8592359/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39