This is not true. While some plugins might work some may not. It depends what hook they use. What sql it uses etc. How ever it is not to hard to update plugins from previous versions of vBulletin for 4.0.
Your best bet is to set up a test board and try it out for your self.
Sorry, I didn't mean to imply that if it didn't have a version checker script it would automatically work. Only that, if it didn't have a version checker script, it wouldn't fail because of the plugin "being the wrong version". Obviously hook locations, syntaxes, etc. change from version to version. I was only trying to point out that you aren't going to get an error when you upload it that says "Plugin was made for xxx version of vBulletin. Actual version is xxx, terminating product." or something similar.