I have samsung galaxy music duos S6012 mobile. according to the specification the internal storage is 4GB but physically my phone shows only 2GB. So, please help what do I do in this manner. I know that my device having fully 4GB storage but it is hidden undauntedly. So pl z help me.
Manufacturers always specify the size of the chip here (4 GB). As that's the bigger number, it's also the better seller – and it's not a straight lie, as it is "internal storage". But the fact they don't put in big letters is, that this storage is split – and only a part of it is available to the user directly.
Details are described in Android Folder Hierarchy here on our site. A short abstract, concentrating on your issue, and for that simplified: "physical internal storage" is split into multiple parts, including...
/system: read-only during "normal operation". This is where the OS is stored, and this is also where all the bloatware is installed (which is what makes it so hard to remove). Especially with Samsung devices, this part often takes up half of the "chip" – which might already explain where your missing 2 GB ended up.
/recovery: two special partitions for technical reasons. Their names already indicate their usage.
/cache: again a speaking name.
/data: This is what is shown as "available space" to the user. Here you can install apps, and they can store their data here.
There's basically nothing you can do to gain additional internal space – unless you want to risk voiding your warranty, and have enough knowledge to "act behind the scenes" (root the device, re-partition internal storage) – which then might prevent system updates (as they might check for the correct partitioning), so you're on your own with those.