It is important to note that the full amount of memories will not be available to RT OS, as some is reserved for other components (e.g. video card, device enumeration, other motherboard requirements). The expected amount is a little less than 3 GB of RAM available to the real-time OS when you use Phar Lap with 4 GB RAM.
LabVIEW Real-Time module installed on your development machine cross-compiles applications for both 32-bit and 64-bit architectures and the application bitness matches the bitness of the RT target distribution. You can monitor memory usage on RT targets
programmatically or using utilities like Measurement and Automation Explorer or Distributed System Manager.