iPhone 5 camera when in use shows a blank screen

First, make sure no running multiple apps and opening the camera at the same time, remove SD card or replace with a new SD card, and factory reset or reroot the device.

Then, check the back camera on Galaxy S4 and ensure its connector not loose or give the camera a few sturdy tap.

Last, if all above the solutions don’t work, you should choose a new rear camera replacement for Galaxy S4. If only the camera lens is scratched or cracked, you can clean it from inside or replace with a new camera lens glass, but make sure to perform this procedure gently, as the glass is quite delicate in nature.

(from here) though it's for back camera, i think it's also suitable with front camera. You can have a try by the steps.

Have a good luck!