However, two readers, John Matthewman (using a PowerBook 1400) and Mark Vogel (using a Duo 280c upgraded to a 2300) both report that the RealPlayer Installer crashed on launch - even if you have all extensions off. Mark cites other reports of this on the comp.sys.mac.comm newsgroup.
Jim Bain claims that RealPlayer 4.0 installs ObjectSupportLib 1.1. However OSL 1.2 is the latest version. You may need to reinstall this after installing RealPlayer.