The best thing to do is first delete any add-ons you don't plan to be using in vB4 (like ones that aren't upgraded) and then disable all of the plugins via first the Manage Products page and then the leftovers via the Plugin Manager. Then, add that line to your config.php file for good measure and perform your upgrade. If your plugins are all disabled, then you won't get any errors like in the first post.
Just a reminder, but please be sure to backup your database prior to performing your upgrade.