Wow, this is really a good news! Thank you so much for sharing this !!!
One more thing, the VirtualBox RAM configuration you are using is 1GB+0.5GB+0.5GB+24MB+1GB, which is less than 4GB, can you configure one of them to 3GB and have a try? This would make sense.
Yes, I can have more than 4GB of RAM assigned across all totality of the VMs. Of course VirtualBox fires up an individual process for each guest that runs, so that's not surprising.