I think we are getting 64-bit microcontrollers because everything else is 64-bit, so 64-bit has become the natural/cost-effective choice. Rather than because uC's are regularly utilizing 4+ GB. It seems unlikely that 4+ GB of RAM will be common just yet.
I wouldn't call the minnowboard a microcontroller, it's more similar to other single board computers like the Pandaboard and the odroid boards. And 2GB are already common for such boards, so 4GB are really not far off.
