I have had the exact progression of issues and coincidentally, this is also with a brand new 64gb 10 Class Samsung Micro SD Pro card. This the second card with this issue, the first being a ScanDisk.
First, I started seeing weird icons where some pictures should have been in one of the folders that I created in the gallery. Yet, I could still see the actual thumbnails correctly. Shortly after noticing this, the thumbnails disappeared and so did the entire "camera" folder in the gallery.
I hooked the phone to the PC and wasn't able to find the camera folder so off to the Samsung kiosk at Best Buy. Their rep didn't offer a reason, but he did "flash" the firmware on the phone and the camera folder reappeared without any of the pictures that I'd taken. The corrupt file icon is still in the other folder that I created, so flashing the firmware didn't resolve the issue. He assured me that the pictures were recoverable but told me that he wasn't allowed to attach my micro sd card to his computer.
I've spent 2 days trying to find a solution, most of it on the phone with Samsung (ticket # 2130661933). That was the most recent ticket number because previous reps kept losing my call and since Samsung won't allow their employees to give you a call back number for them if the call is lost, you have to start from scratch.
As with the previous post, my issues began over a period of time and ended with only recent photos being corrupted. One rep blamed non-Samsung cards. Clearly not applicable in my case. Another rep from the remote access department actually admitted that the GS4 hardware is not fully compatible with Kitkat which needs a more beefed up phone (S5) in order to run efficiently,
Needless to say, I got nowhere with Samsung and ZAR and chkdisk weren't able to recover the photos that are damaged. Verizon is sending me another S4, but if that has Kitkat installed, I'm SOL.
Please, PLEASE escalate all issues to Samsung (feel free to reference my ticket number) as well as the carrier because they obviously don't share information or aren't truly interested in fixing the problem. I'd also like to know if someone finds a solution.
Thanks and good luck

to everyone.