Here we go with a possible fix,,,,
I lost both cameras after going to ios 7, I waited for ages to see if there were any problems with my wifes ios upgrade, then switched from ios 6 to ios 7 last week, we both have iPhone 4 16gb devices.
It was only recently that I found out how to close apps that are open in the background which I believe zap some of the power, (RAM in pc terms), on ios 6 It was very different to how you do this on ios 7.
THE FIX? Maybe,,,
Double press the home button, this will show a line across the bottom of the screen of the icons for apps that are still running AND a small image of the app above the icon, close all of these one by one, by swiping the image (not the icon) upwards, once you have closed them all, power down the device.
Restart the device by holding the home button and power button for 10 seconds the apple logo comes on, then off, then on again at the 10 second mark, release both buttons and leave the device to fully start.
At that point I tried my camera and they both worked.
I don't know if this has been tried or talked about before but it has solved my camers issues and I wanted to share in case it is the fix.
Response to the result of the above would be interesting and welcome.