I think 8GB is necessary to avoid the OS from using its paging file.
Hum... I have yet to have a computer use its pagefile with 4GB of RAM installed. That's with my work VM running with 1.5 GB configured to it.
Of course looking at one of our SAP servers, it has over 48 GB of RAM in use, but then again, I don't run a full blown SAP CI on my MBA