For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “increase”, there isn't any much more letters afterwards and thus have to be the word "include". The word “addict” is within the BIP39 wordlist but you would've applied “addi” for “addict”. You https://tornadosocial.com/story4566012/ledger-seed-phrase-bip39-no-further-a-mystery