By “monthly membership” do you mean access to these forums, or paying $10/month for access to PS+LR, the Photography Plan?
If the latter, realize that you cannot get an earlier version of LR than 5 with the monthly plan, because LR4 had no ability to check to see if it is licensed via a subscription. This capability started with LR 5.
If you mean you need to buy LR4 or upgrade your computer to OS X 10.7+ or Windows7+, then your only option with your current computer is to buy the obsolete LR4 on somewhere like Amazon, and make sure it is the full version not the upgrade version.
I mean the latter. I will try to find LR 4 then. Thanks!
oh and so then how do I return Lightroom 5?
I’m not sure because I’ve never returned an Adobe products, but Googling came up with this Adobe Help page about various scenarios: