...Until a version of OS X comes out that supports that generation of Mac but requires a minimum of 8GB...
Not being able to run the latest version of OS X isn't a problem, but it's only a matter of time before you're two behind and at that point, you stop being able to receive updates to basic things like Safari and Adobe Flash Player. Being able to run the latest version of OS X forestalls that. Apple has been, with the exception of Mountain Lion, been doubling the minimum RAM requirement from 10.3 through 10.7. It's safe to say that, especially with Mountain Lion abandoning the 32-bit kernel, they wil up the RAM requirement again soon. So yeah, future proofing, it's a real consideration.
I doubt there will be OSX upgrades that will require 8GB's of RAM minimum in the period the OP is going to use the computer. 8GB's of RAM is just like insurance in this case.