Re: Windows 8.1 for iMac 2009
I think you might be confusing Boot Camp Assistant - the Mac app that enables you to partition your Mac hard drive with a Windows partition and get started on the Windows installation - with the actual support software that gets installed when you're running Windows.
That support software is a Windows installer executable you run after you've initially installed Windows, which enables Windows to do things like support your Mac's AirPort card, graphics, Bluetooth and other key hardware elements. It's vital to get your Mac working with Windows properly.
Here's an Apple knowledgebase article with the pertinent details about what Macs support which version of Windows:
Boot Camp: System requirements for Microsoft Windows operating systems
As you can see from the iMac chart on that page, Apple says you need to use Boot Camp Support software 4.0.4033 on that Mac, and that version only supports Windows 7.
Again, using a virtualization app like Parallels Desktop or VMware Fusion won't impede you from installing whatever version of Windows (or any other x64) operating system you want to use; I have a virtual machine running Elementary OS, a Ubuntu Linux derivative, on mine. There are some speed penalties for using a virtual machine instead of Boot Camp and there's an added cost, but you don't have to reboot your Mac to use it, and based on your use case (an Access database) I don't think it'll be a big deal.
The 4 GB of RAM will be perfectly fine.