One thing you "should" be able to do is just zip up a working build 509 and unzip it onto your system (for that matter, probably just the terminal.exe file is all you need). MT4 can usually run from anywhere in my experience. Even if you acquire it from a different broker, you should still be able to change the server addresses to work with your broker.

But I thought MT4 had forced updates now -- doesn't that mean it would automatically upgrade itself upon startup of build 509? Or was there only 1 forced updated to build 500?