You can currently only buy MC5.5 or MC6 from Avid. They don't sell upgrades to older versions.
However, if you have XpressPro, you probably have a dongle for that. If you upgrade that dongle to the current version of Media Composer for 599, you can subsequently use that dongle to launch any older installed version of Media Composer. So an MC6 dongle will allow you to run MC4, MC5, any version.
If you buy the upgrade you will receive an activation code (for the newer software licensing system), but you can trade in that license key for a dongle updater at avid.com.upgrade. This gets you an .avd dongle updater file. You then open the Avid Dongle Manager on your system, with the dongle attached, and you update your dongle with the .avd file (done in less than a minute).
Also consider that there is a limited-time offer to upgrade your Xpress Pro or Media Composer to Symphony-software, which includes Boris Continuum Complete 8, at 995. A super sweet deal.
If you have a Final Cut Pro license, you can even use that to crossgrade to Symphony with BCC8, also for 995, and then you get to keep your Xpress Pro (and FCP) as well. See
http://www.avid.com/symphonypromo for details.