My 16gb Galaxy Tab 4 is showing too much storage used, but doesn't show what's using it all

[Beat Breaker]

I have a tab 4 16gb, and it shows out of 12gb storage available, 8.7gb is used... yet when I use the app managers, and file explorers, I am only able to locate approximately 4.2 gb Total of files on the internal memory. I know that 4gb goes to the system, which is not shown, but that's accounted for by it only saying 12gb available. Why does it say I have almost 9gb used of my available 12gb... how do I find and remove whatever is eating up all my extra space? App suggestions are appreciated to help resolve this. I only have 2 large apps installed, which were 1.1gb and 1.2gb, but even they show only 292mb, and 287mb in the data folder. Are the downloaded apk files hidden somewhere in my system, and can I delete those without removing the app? I am on updated android lollipop 5.1.1, stock. Please help, I have a 32gb SD card installed and would love to move whatever I can to it to get my space back. Disk usage apps say only 4.92gb belong to apps/data folder.