User ultrasawblade may be right, but not necessarily.
I have a 16 GB USB flash stick which has two compulsory partitions on it. They don't seem to be partitions in the traditional sense, and can only be resized using a utility that was included on the stick.
There's no way to get rid of the unwanted partition at all, which was described as for some kind of encryption purposes that I don't need. All I could do was to shrink the unwanted partition as much as possible, so as much space as possible is given to the wanted partition, then tell Windows not to mount the unwanted partition (no drive letter or other mount point) in Computer Management - Disk Management.
This was a USB stick that I paid for as a 16 GB USB stick, and with no warning of this "feature", and this was back when 16 GB USB sticks were only just becoming available.
For this reason, plus the incredibly slow performance compared with other even older flash memory (early MLC flash, I assume, but again there was no warning) I advise people to avoid "integral" USB flash sticks. But that's about all that it's worth my while to do.
Basically, if some company wants to distribute a flash device with some forcibly reserved space, that's not expecially hard for them to do. If it's possible to reconfigure the device at all, it may require software that you don't have access to.
There's at least a chance that your only resolution would be legal rather than technical - to argue to the retailer or, as a last resort, in court that you paid for the phone on the understanding that you would get a 4 GB microSD card with all of the space available for your purposes. In the UK, the relevant consumer laws would relate to "fitness for purpose". Calling something a "free" extra/gift on the packaging doesn't legally exclude it. To be safe, though, you must tell the retailer your purpose before you pay for the item, and even then it's very likely your word against theirs.
In any case, consumer laws vary a lot from country to country.
And of course, the retailer knows you're unlikely to go to court over a flash card that can be replaced for so little. That's a standard get-out for consumer law.
On the other hand, the longer you're at the retailers complaining, the more sales they lose.