Bip39 Vs Slip39. The seed phrase is usually converted to some amount which can be used given that the seed integer to your deterministic wallet that generates all the essential pairs used in the wallet. For words that only have 3 letters, there is absolutely no 4th letter. For example https://cypriotdirectory.com/listings13169822/bip39-passphrase-generator-https-linktr-ee-bip39-the-smart-trick-of-bip39-list-that-nobody-is-discussing