Android 4.1.1 system is consuming 4GB of my memory?
Yes, you have an 8GB tablet.  How Android partitioning works is there is a NAND flash chip, and then it's basically split up into system storage (/system) and otherwise, and user accessible storage (/data) and possibly (in some cases), /sdcard (note that I'm talking about the emulated legacy SD Card, not your expansion slot.

So, when they say it's an 8GB tablet, they're correct.

EDIT:

Looking at your df output...  Blksize refers to the maximum length, in bytes, of a file storage block.  You don't have to worry about that column.  What we're focused on is all the other partitions.  Let's start at the top:

/dev contains device nodes.  Don't worry about the reported size - it has no effect on your storage.

/mnt/asec and /mnt/obb are each partitioned with 443.2MB.  With those two partitions (required by the system), you have 886.4MB, rounding up.

/system (where Android lives) is partitioned at 607.4MB, which will bring our total up to 1.5GB.

/data is where all user preferences and settings are stored.  When you do a factory reset, you are wiping the /data partition.  Adding in the 1.2GB from there, we have a total of 2.7GB.

/cache is also required by Android, and with a 629.9MB partition, we're at 3.3GB.

/mnt/external_sd is your external SD card.  Forget about that for now.

/mnt/sdcard is the internal SD card - a partition of user accessible storage in the internal memory.  It's set at 4.5GB - which is what your vendor is reporting.

Add all these partitions together, and we have a total of 7.8GB which is correct for an 8GB tablet.

Storage manufacturers report storage in decimal, meaning 1000MB = 1GB.  However, in binary, 1024MB = GB, so that's why you have a discrepancy.