neither IE8 nor IE9 are supported in RichFaces 4.5 but it doesn't mean that it won't work. We definitely know that rich:fileUpload doesn't work in IE8 and IE9 because of missing API in those browsers but the majority of components should work fine. It's up to you to try it out. Branches 4.2 and 4.3 are no longer maintained so the best would be to upgrade to 4.5.
Should you find any problem with older IE, feel free to submit a pull request with a fix