I have got a Samsung S3 that forces me to use a 6-charakter password for the phone/SD card encryption. I would like to use a PIN (4-digits) as it was possible in earlier Samsung phones. I am aware of the reduced security but for practical reasons I want to use the PIN. What are my options? I see there is the option to use any kind of "file vault" app, but they require to move my files and folders into the apps container. Is there any option to encrypt the android leaving the files in the original place?
You already indicated the possible use of an app. And indeed, next to the so-called "vault apps", there are several options for Encryption of Files and Folders which work transparently on the file system. For some, you still will need to use "specific folders" which you encrypt, though – while most of the others fall into the category of requiring root access.
Some example apps you might wish to take a closer look at include, but are not limited to: