You're going to have to try to port it to vB4.2.2 or hire someone to do it for you if you still want to use it.
One of the things to consider before "upgrading" when you are making the pros and cons list, looking for those compelling reasons to do so I keep talking about.
Also just as Max Taxable said you also need to put on that list every mod you have currently installed on your forum, because NONE of them will work with vB4. So they would need to be uninstalled and a new vB4 version would need to be installed. And not all of the vB3 mods were ported to vB4 so you may have functions of your site that cannot be upgraded at all. It can be quite a big headache.
Not true. There's quite a number of addons for vB3 that flawlessly work in vB4. You also don't have to uninstall addons, you deactivate them for upgrade, then update them. For many addons you'd lose data if you uninstall them.