For words that have only 3 letters, there's no 4th letter. As an example with the word “insert”, there won't be any far more letters afterwards and thus must be the word "increase". The word “addict” is in the BIP39 wordlist but you might have applied “addi” for “addict”. Artwork https://socialclubfm.com/story9538457/an-unbiased-view-of-bip39-seed-phrase-github