Why don't they make wood cutoff blades for 4.5" grinder?
They are made, they look like mini chainsaws and are commonly used for carving bowls.



A common brand for these blades is Lancelot

You can also get smaller saw blades in the 4.5" form factor. I suggest you go to a saw store or woodworking shop. Here is a product on Amazon:

http://www.amazon.com/KwikTool-BB450-2-Inch-1-Inch-8-Inch/dp/B000O3IOEW/ref=sr_1_2?ie=UTF8&amp;qid=1357252670&amp;sr=8-2&amp;keywords=angle+grinder+wood+blade