Android 4.3 has been leaked for the European version of the Samsung Galaxy S4 model (GT-i9505). Those who are lucky enough to use this update say that it is faster than Android 4.2.2 and it is lag free. Samsung Knox is present along with Samsung Wallet.
Other than that it contains a reading mode which make screen look better, color quality has been improved with a sharper text result and a new QWERTY keyboard with some minor UI changes.
The Korean OEM is believed to be thinking to add some other features and update to it and release it this month or may be next month.