It sounds as though you have a lot to learn about computers in general.
#1 what does USB have to do with internal memory ?
#2 What does a "memory card" have to do with internal memory ?
#3 What is a "memory card" ? We could presume you mean an SDCARD, but that's not the context of your quesitons.
So, internal memory *is* 512MB( Megabyte ) or 4Gbit( Gigabit ) but even free wont show 512MB total system ram. Why ? I'm not 100% positive. But my guess would be part of the total system memory is mapped to the video frame buffer device on the processor.
Passed that, the Beaglebone black comes with an Debian image that is loaded with features. As such, 4GB in terms of storing a full blow GUI desktop, and many demo apps is not a lot of space.
Additionally, if the board you're using comes with a 2 partition setup. The shared partition is only ~100MB total in size. It's also an MSDOS, or FAT partition, which Linux does not run on. Linux runs on the second partition.
Now if you are wondering why this and that. I suggest you find a book such as derrek molloy's "Exploring beaglebone black" and start reading.