The answer, in my case, is probably not. At least for now.
On the get-more-RAM side: I do use a several virtual machines (VMs) regularly. With eight gigabytes, I could open all of them at once, and increase the RAM available to each of them too, and still have plenty of RAM left over for other purposes. I could also run Windows 7 in a VM, something that might be problematic with only 4GB.
On the not-right-now side: in my current setup, so long as I don’t open more than two virtual machines at a time, I don’t run into memory problems. And the current allocation of memory to each VM, while not massive, seems sufficient to prevent most swapping within the VM itself. But the biggest argument against it is the cost: replacing my current 4GB with 8GB on this machine would run about $750 from Newegg.com (the least expensive option I’ve been able to find so far). The advantages aren’t worth that to me.
On the other hand, electronics prices almost always drop over time. If this stuff drops fast enough, or if I find that Project X needs a lot of RAM to operate at full speed (possible, I’m not sure yet), I might end up getting it. We’ll see.
Of course, the answer might be different for you, especially as memory for most computers is a lot cheaper than for mine. Read the article and make your own determination.