Try this if you aare not comfortable with the Sawsall.
King Arthur's Tools 45822 Lancelot 22 tooth, 5/8" center hole - Power Angle Grinders - Amazon.com
It will fit in your 4.5" grinder but you almost have to do it without a guard. Its a great little tool, but I think its a bit dangerous. With a Sawsall, worst is you WILL bent a few blades getting going. Once you get it going, it should go quick. 15 minutes, it should be done.