Vodafone has started rolling out the September security patch for the Galaxy S4 Mini in Europe, which leads us to believe that Samsung will follow suit in the near future with a similar update for the factory unlocked variant of the handset. As outlined in the changelog posted on the firm's Security Blog almost two months ago, the upgrade transports a total of 66 patches for the Android OS.
In typical fashion, Vodafone is distributing the firmware in stages across different regions. However, seeing as there aren't too many people with a Galaxy S4 Mini operating on the network, you shouldn't have to wait too long to start installing it. To see if it's ready for your device, heading into Settings » About Device » Software Update » Update Now.