For words that only have three letters, there isn't any 4th letter. Such as with the word “increase”, there aren't any far more letters Later on and thus need to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would have employed “addi” for “addict”. https://fellowfavorite.com/story20032194/ledger-seed-phrase-bip39-no-further-a-mystery