No, you don't need both of them in the same machine and the 6.0 won't replace the 4.0. You can uninstall the 4.0 without hurting the 6.0.
I recently downloaded the free version of adobe reader 6.0 and I already had adobe acrobat 4.0 . Do I need both in my system, or does the 6.0 replace the 4.0? If so, should I un-install/delete the 4.0? I also have the adobe manager that came with the 6.0. I appreciate the help, because all of you guys are always ready to jump in and answer/help. Thanks, Tom..