the roto-zip is about five years old if that and it really is a beefed up dremel with many other features. You can get the cut off saw feature and set the depth and do what you want with about 4 inches of clearance.
This tool is great if its Not in your arsenal.
Then you can get that mini circular saw that might work. It has a 4 inch diameter blade. Might work, but I would prefer the rotozip.